Add FlexDLL as git submodule

Messages for obtaining flexdll sources updated in Makefile.nt

.gitignore

@@ -294,6 +294,3 @@
 /yacc/ocamlyacc
 /yacc/version.h
 /yacc/.gdb_history
-
-# Bootstrapped FlexDLL
-flexdll/

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "flexdll"]
+	path = flexdll
+	url = https://github.com/alainfrisch/flexdll.git

.travis.yml

@@ -12,6 +12,8 @@
 
 sudo: false
 language: c
+git:
+  submodules: false
 script: bash -ex .travis-ci.sh
 matrix:
   include:

Makefile.nt

@@ -18,11 +18,22 @@ include Makefile.shared
 defaultentry:
 	@echo "Please refer to the installation instructions in file README.win32.adoc."
 
-# FlexDLL sources missing error message
-flexdll/Makefile:
-	@echo In order to bootstrap FlexDLL, you need to place the sources in flexdll.
+# FlexDLL sources missing error messages
+# Different git mechanism displayed depending on whether this source tree came
+# from a git clone or a source tarball.
+
+# Displayed in all cases
+flexdll-common-err:
+	@echo In order to bootstrap FlexDLL, you need to place the sources in flexdll
 	@echo This can either be done by downloading a source tarball from
 	@echo \  http://alain.frisch.fr/flexdll.html
+
+flexdll/Makefile: $(if $(wildcard flexdll/Makefile),,$(if $(wildcard .git),flexdll-common-err,flexdll-repo))
+	@echo or by checking out the flexdll submodule with
+	@echo \  git submodule update --init
+	@false
+
+flexdll-repo: flexdll-common-err
 	@echo or by cloning the git repository
 	@echo \  git clone https://github.com/alainfrisch/flexdll.git
 	@echo
@@ -716,6 +727,6 @@ distclean:
 .PHONY: ocamltoolsopt.opt ocamlyacc opt-core opt opt.opt otherlibraries
 .PHONY: otherlibrariesopt promote promote-cross
 .PHONY: restore runtime runtimeopt makeruntimeopt world world.opt
-.PHONY: flexdll flexlink_opt
+.PHONY: flexdll flexlink_opt flexdll-common-err flexdll-repo
 
 include .depend

flexdll

@@ -0,0 +1 @@
+Subproject commit db1d82aa8662d146562067288d0331c4ec2e1bd0