diff --git a/Makefile b/Makefile
index 0632dece5..a4379a44f 100644
--- a/Makefile
+++ b/Makefile
@@ -283,15 +283,18 @@ BOOT_FLEXLINK_CMD=
 ifeq "$(UNIX_OR_WIN32)" "win32"
 FLEXDLL_SUBMODULE_PRESENT := $(wildcard flexdll/Makefile)
 ifeq "$(FLEXDLL_SUBMODULE_PRESENT)" ""
-  BOOT_FLEXLINK_CMD=
-  FLEXDLL_DIR=
+  BOOT_FLEXLINK_CMD =
+  FLEXDLL_DIR =
+  FLEXLINK_ENV =
 else
   BOOT_FLEXLINK_CMD = FLEXLINK_CMD="../boot/ocamlrun ../flexdll/flexlink.exe"
-  CAMLOPT := OCAML_FLEXLINK="boot/ocamlrun flexdll/flexlink.exe" $(CAMLOPT)
+  # FLEXLINK_ENV must have a space at the end
+  FLEXLINK_ENV = OCAML_FLEXLINK="boot/ocamlrun flexdll/flexlink.exe" #
   FLEXDLL_DIR=$(if $(wildcard flexdll/flexdll_*.$(O)),+flexdll)
 endif
 else
-  FLEXDLL_DIR=
+  FLEXDLL_DIR =
+  FLEXLINK_ENV =
 endif
 
 # The configuration file
@@ -879,7 +882,7 @@ partialclean::
 
 ocamlc.opt: compilerlibs/ocamlcommon.cmxa compilerlibs/ocamlbytecomp.cmxa \
             $(BYTESTART:.cmo=.cmx)
-	$(CAMLOPT) $(LINKFLAGS) -o $@ $^ -cclib "$(BYTECCLIBS)"
+	$(FLEXLINK_ENV)$(CAMLOPT) $(LINKFLAGS) -o $@ $^ -cclib "$(BYTECCLIBS)"
 
 partialclean::
 	rm -f ocamlc.opt
@@ -893,7 +896,7 @@ partialclean::
 
 ocamlopt.opt: compilerlibs/ocamlcommon.cmxa compilerlibs/ocamloptcomp.cmxa \
               $(OPTSTART:.cmo=.cmx)
-	$(CAMLOPT) $(LINKFLAGS) -o $@ $^
+	$(FLEXLINK_ENV)$(CAMLOPT) $(LINKFLAGS) -o $@ $^
 
 partialclean::
 	rm -f ocamlopt.opt
@@ -1286,7 +1289,7 @@ ocamlnat$(EXE): compilerlibs/ocamlcommon.cmxa compilerlibs/ocamloptcomp.cmxa \
     compilerlibs/ocamlbytecomp.cmxa \
     compilerlibs/ocamlopttoplevel.cmxa \
     $(OPTTOPLEVELSTART:.cmo=.cmx)
-	$(CAMLOPT) $(LINKFLAGS) -linkall -o $@ $^
+	$(FLEXLINK_ENV)$(CAMLOPT) $(LINKFLAGS) -linkall -o $@ $^
 
 partialclean::
 	rm -f ocamlnat$(EXE)
diff --git a/lex/Makefile b/lex/Makefile
index 8e8b0fd43..0121a6a61 100644
--- a/lex/Makefile
+++ b/lex/Makefile
@@ -21,9 +21,11 @@ CAMLYACC ?= ../yacc/ocamlyacc
 ROOTDIR=..
 
 ifeq "$(wildcard $(ROOTDIR)/flexdll/Makefile)" ""
-export OCAML_FLEXLINK:=
+FLEXLINK_ENV =
 else
-export OCAML_FLEXLINK:=$(ROOTDIR)/boot/ocamlrun $(ROOTDIR)/flexdll/flexlink.exe
+# FLEXLINK_ENV must have a space at the end
+FLEXLINK_ENV = \
+  OCAML_FLEXLINK="$(ROOTDIR)/boot/ocamlrun $(ROOTDIR)/flexdll/flexlink.exe" #
 endif
 
 CAMLC=$(CAMLRUN) ../boot/ocamlc -strict-sequence -nostdlib -I ../boot \
@@ -49,7 +51,7 @@ ocamllex: $(OBJS)
 	$(CAMLC) $(LINKFLAGS) -compat-32 -o ocamllex $(OBJS)
 
 ocamllex.opt: $(OBJS:.cmo=.cmx)
-	$(CAMLOPT) -o ocamllex.opt $(OBJS:.cmo=.cmx)
+	$(FLEXLINK_ENV)$(CAMLOPT) -o ocamllex.opt $(OBJS:.cmo=.cmx)
 
 clean::
 	rm -f ocamllex ocamllex.opt
diff --git a/ocamldoc/Makefile b/ocamldoc/Makefile
index 376702374..db8960af2 100644
--- a/ocamldoc/Makefile
+++ b/ocamldoc/Makefile
@@ -23,15 +23,17 @@ OCAMLYACC ?= $(ROOTDIR)/yacc/ocamlyacc
 
 STDLIBFLAGS = -nostdlib -I $(ROOTDIR)/stdlib
 OCAMLC    = $(OCAMLRUN) $(ROOTDIR)/ocamlc $(STDLIBFLAGS)
-ifeq "$(UNIX_OR_WIN32)" "unix"
 OCAMLOPT  = $(OCAMLRUN) $(ROOTDIR)/ocamlopt $(STDLIBFLAGS)
+ifeq "$(UNIX_OR_WIN32)" "unix"
+FLEXLINK_ENV =
 else # Windows
   ifeq "$(wildcard $(ROOTDIR)/flexdll/Makefile)" ""
-    FLEXLINK_ENV=
+  FLEXLINK_ENV =
   else
-    FLEXLINK_ENV=OCAML_FLEXLINK="$(ROOTDIR)/boot/ocamlrun $(ROOTDIR)/flexdll/flexlink.exe"
+  # FLEXLINK_ENV must have a space at the end
+  FLEXLINK_ENV = \
+    OCAML_FLEXLINK="$(ROOTDIR)/boot/ocamlrun $(ROOTDIR)/flexdll/flexlink.exe" #
   endif
-  OCAMLOPT = $(FLEXLINK_ENV) $(OCAMLRUN) $(ROOTDIR)/ocamlopt $(STDLIBFLAGS)
 endif
 OCAMLDEP  = $(OCAMLRUN) $(ROOTDIR)/tools/ocamldep -slash
 OCAMLLEX  = $(OCAMLRUN) $(ROOTDIR)/boot/ocamllex
@@ -216,7 +218,7 @@ $(OCAMLDOC): $(EXECMOFILES)
 	$(OCAMLC) -o $@ -linkall $(LINKFLAGS) $(OCAMLDOC_BCLIBRARIES) $^
 
 $(OCAMLDOC_OPT): $(EXECMXFILES)
-	$(OCAMLOPT) -o $@ -linkall $(LINKFLAGS) $(OCAMLDOC_NCLIBRARIES) $^
+	$(FLEXLINK_ENV)$(OCAMLOPT) -o $@ -linkall $(LINKFLAGS) $(OCAMLDOC_NCLIBRARIES) $^
 
 $(OCAMLDOC_LIBCMA): $(LIBCMOFILES)
 	$(OCAMLC) -a -o $@ $(LINKFLAGS) $^
@@ -268,7 +270,7 @@ odoc_see_lexer.ml: odoc_see_lexer.mll
 	$(OCAMLOPT) $(OCAMLPP) $(COMPFLAGS) -c $<
 
 .ml.cmxs:
-	$(OCAMLOPT) -shared -o $@ $(OCAMLPP) $(COMPFLAGS) $<
+	$(FLEXLINK_ENV)$(OCAMLOPT) -shared -o $@ $(OCAMLPP) $(COMPFLAGS) $<
 
 .mll.ml:
 	$(OCAMLLEX) $<
diff --git a/ocamltest/Makefile b/ocamltest/Makefile
index 527ee0c9d..c6863b1ee 100644
--- a/ocamltest/Makefile
+++ b/ocamltest/Makefile
@@ -22,15 +22,16 @@ ifeq "$(UNIX_OR_WIN32)" "win32"
   ocamlsrcdir := $(shell echo "$(abspath $(shell pwd)/..)"|cygpath -w -f - \
     | sed 's/\\/\\\\\\\\/g')
   ifeq "$(wildcard ../flexdll/Makefile)" ""
-    FLEXLINK_ENV=
+    FLEXLINK_ENV =
   else
-    FLEXLINK_ENV=OCAML_FLEXLINK="../boot/ocamlrun ../flexdll/flexlink.exe"
+    # FLEXLINK_ENV must have a space at the end
+    FLEXLINK_ENV = OCAML_FLEXLINK="../boot/ocamlrun ../flexdll/flexlink.exe" #
   endif
   mkexe := $(MKEXE_ANSI) -link $(OC_LDFLAGS)
 else
   unix := true
   ocamlsrcdir := $(abspath $(shell pwd)/..)
-  FLEXLINK_ENV=
+  FLEXLINK_ENV =
   mkexe := $(MKEXE)
 endif
 
@@ -158,9 +159,9 @@ flags := -g -nostdlib $(include_directories) \
   -strict-sequence -safe-string -strict-formats \
   -w +a-4-9-41-42-44-45-48 -warn-error A
 
-ocamlc := $(FLEXLINK_ENV) ../runtime/ocamlrun ../ocamlc $(flags)
+ocamlc := ../runtime/ocamlrun ../ocamlc $(flags)
 
-ocamlopt := $(FLEXLINK_ENV) ../runtime/ocamlrun ../ocamlopt $(flags)
+ocamlopt := ../runtime/ocamlrun ../ocamlopt $(flags)
 
 ocamldep := ../runtime/ocamlrun ../tools/ocamldep -slash
 
@@ -178,13 +179,13 @@ allopt: ocamltest.opt$(EXE)
 opt.opt: allopt
 
 ocamltest$(EXE): $(bytecode_modules)
-	$(ocamlc) -custom ocamlcommon.cma -o $@ $^
+	$(FLEXLINK_ENV)$(ocamlc) -custom ocamlcommon.cma -o $@ $^
 
 %.cmo: %.ml
 	$(ocamlc) -c $<
 
 ocamltest.opt$(EXE): $(native_modules)
-	$(ocamlopt) ocamlcommon.cmxa -o $@ $^
+	$(FLEXLINK_ENV)$(ocamlopt) ocamlcommon.cmxa -o $@ $^
 
 %.cmx: %.ml
 	$(ocamlopt) -c $<
diff --git a/otherlibs/Makefile b/otherlibs/Makefile
index 22a4806c6..d3b8a4397 100644
--- a/otherlibs/Makefile
+++ b/otherlibs/Makefile
@@ -22,9 +22,11 @@ include $(ROOTDIR)/Makefile.common
 CAMLRUN ?= $(ROOTDIR)/boot/ocamlrun
 
 ifeq "$(wildcard $(ROOTDIR)/flexdll/Makefile)" ""
-export OCAML_FLEXLINK:=
+FLEXLINK_ENV =
 else
-export OCAML_FLEXLINK:=$(ROOTDIR)/boot/ocamlrun $(ROOTDIR)/flexdll/flexlink.exe
+# FLEXLINK_ENV must have a space at the end
+FLEXLINK_ENV = \
+  OCAML_FLEXLINK="$(ROOTDIR)/boot/ocamlrun $(ROOTDIR)/flexdll/flexlink.exe" #
 endif
 
 CAMLC=$(CAMLRUN) $(ROOTDIR)/ocamlc -nostdlib -I $(ROOTDIR)/stdlib
@@ -74,10 +76,11 @@ $(LIBNAME).cmxa: $(CAMLOBJS_NAT)
 	         $(CAMLOBJS_NAT) $(LINKOPTS)
 
 $(LIBNAME).cmxs: $(LIBNAME).cmxa lib$(CLIBNAME).$(A)
-	$(CAMLOPT) -shared -o $(LIBNAME).cmxs -I . $(LIBNAME).cmxa
+	$(FLEXLINK_ENV)$(CAMLOPT) -shared -o $(LIBNAME).cmxs -I . \
+	                                     $(LIBNAME).cmxa
 
 lib$(CLIBNAME).$(A): $(COBJS)
-	$(MKLIB) -oc $(CLIBNAME) $(COBJS) $(LDOPTS)
+	$(FLEXLINK_ENV)$(MKLIB) -oc $(CLIBNAME) $(COBJS) $(LDOPTS)
 
 install::
 	if test -f dll$(CLIBNAME)$(EXT_DLL); then \
diff --git a/otherlibs/systhreads/Makefile b/otherlibs/systhreads/Makefile
index ae9a7a6ce..94fef78c0 100644
--- a/otherlibs/systhreads/Makefile
+++ b/otherlibs/systhreads/Makefile
@@ -21,9 +21,11 @@ include $(ROOTDIR)/Makefile.common
 CAMLRUN ?= $(ROOTDIR)/boot/ocamlrun
 
 ifeq "$(wildcard $(ROOTDIR)/flexdll/Makefile)" ""
-export OCAML_FLEXLINK:=
+FLEXLINK_ENV =
 else
-export OCAML_FLEXLINK:=$(ROOTDIR)/boot/ocamlrun $(ROOTDIR)/flexdll/flexlink.exe
+# FLEXLINK_ENV must have a space at the end
+FLEXLINK_ENV = \
+  OCAML_FLEXLINK="$(ROOTDIR)/boot/ocamlrun $(ROOTDIR)/flexdll/flexlink.exe" #
 endif
 
 LIBS = -nostdlib -I $(ROOTDIR)/stdlib -I $(ROOTDIR)/otherlibs/$(UNIXLIB)
@@ -67,10 +69,10 @@ all: lib$(LIBNAME).$(A) $(LIBNAME).cma $(CMIFILES)
 allopt: lib$(LIBNAME)nat.$(A) $(LIBNAME).cmxa $(CMIFILES)
 
 lib$(LIBNAME).$(A): $(BYTECODE_C_OBJS)
-	$(MKLIB) -o $(LIBNAME) $(BYTECODE_C_OBJS) $(PTHREAD_LINK)
+	$(FLEXLINK_ENV)$(MKLIB) -o $(LIBNAME) $(BYTECODE_C_OBJS) $(PTHREAD_LINK)
 
 lib$(LIBNAME)nat.$(A): $(NATIVECODE_C_OBJS)
-	$(MKLIB) -o $(LIBNAME)nat $^
+	$(FLEXLINK_ENV)$(MKLIB) -o $(LIBNAME)nat $^
 
 $(LIBNAME).cma: $(THREADS_BCOBJS)
 ifeq "$(UNIX_OR_WIN32)" "unix"
diff --git a/tools/Makefile b/tools/Makefile
index 77f9c1192..bdc547308 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -50,7 +50,8 @@ $1: $3 $2
 	$$(CAMLC) $$(LINKFLAGS) -I .. -o $$@ $2
 
 $1.opt: $3 $$(call byte2native,$2)
-	$$(CAMLOPT) $$(LINKFLAGS) -I .. -o $$@ $$(call byte2native,$2)
+	$$(FLEXLINK_ENV)$$(CAMLOPT) $$(LINKFLAGS) -I .. -o $$@ \
+	                            $$(call byte2native,$2)
 
 all: $1
 
@@ -73,20 +74,16 @@ endef
 ROOTDIR=..
 
 ifeq "$(wildcard $(ROOTDIR)/flexdll/Makefile)" ""
-export OCAML_FLEXLINK:=
+FLEXLINK_ENV =
 else
-export OCAML_FLEXLINK:=$(ROOTDIR)/boot/ocamlrun $(ROOTDIR)/flexdll/flexlink.exe
+# FLEXLINK_ENV must have a space at the end
+FLEXLINK_ENV = \
+  OCAML_FLEXLINK="$(ROOTDIR)/boot/ocamlrun $(ROOTDIR)/flexdll/flexlink.exe" #
 endif
 
 CAMLC=$(CAMLRUN) ../boot/ocamlc -g -nostdlib -I ../boot \
       -use-prims ../runtime/primitives -I ..
 CAMLOPT=$(CAMLRUN) ../ocamlopt -g -nostdlib -I ../stdlib
-ifeq "$(UNIX_OR_WIN32)" "win32"
-  ifneq "$(wildcard ../flexdll/Makefile)" ""
-    CAMLOPT := OCAML_FLEXLINK="../boot/ocamlrun ../flexdll/flexlink.exe" \
-      $(CAMLOPT)
-  endif
-endif
 CAMLLEX=$(CAMLRUN) ../boot/ocamllex
 INCLUDES=-I ../utils -I ../parsing -I ../typing -I ../bytecomp -I ../asmcomp \
          -I ../middle_end -I ../middle_end/base_types -I ../driver \
@@ -366,7 +363,7 @@ LINTAPIDIFF=../compilerlibs/ocamlcommon.cmxa \
 
 lintapidiff.opt: INCLUDES+= -I ../otherlibs/str
 lintapidiff.opt: $(LINTAPIDIFF)
-	$(CAMLOPT) $(LINKFLAGS) -I .. -o $@ $(LINTAPIDIFF)
+	$(FLEXLINK_ENV)$(CAMLOPT) $(LINKFLAGS) -I .. -o $@ $(LINTAPIDIFF)
 clean::
 	rm -f -- lintapidiff.opt lintapidiff.cm? lintapidiff.o