diff --git a/libjs.make b/libjs.make
index da8f17d3..e3a24c98 100644
--- a/libjs.make
+++ b/libjs.make
@@ -1,8 +1,14 @@
 #
 # This makefile is used to build the Javascript dependency for Oolite
 #
-# It can be used to make both a release Javascript library and a
-# debug library.
+# This Makefile is used to download and build the Javascript library
+# for use by Oolite.
+# Depending on invocation, a debug or release (default) version of the
+# library will be built.
+#
+# To use:
+# $ make -f libjs.make debug=(yes|no)
+
 include config.make
 
 LIBJS_SRC_DIR                    = deps/Cross-platform-deps/mozilla/js/src
@@ -13,7 +19,7 @@ ifeq ($(OO_JAVASCRIPT_TRACE),yes)
     LIBJS_CONFIG_FLAGS           += --enable-trace-jscalls
 endif
 ifeq ($(debug),yes)
-    LIBJS_BUILD_DIR=$(LIBJS_SRC_DIR)/build-debug
+    LIBJS_BUILD_DIR              = $(LIBJS_SRC_DIR)/build-debug
     LIBJS_CONFIG_FLAGS           += --enable-debug
     LIBJS_CONFIG_FLAGS           += --disable-optimize
     LIBJS_BUILD_FLAGS            =
@@ -22,20 +28,46 @@ else
     LIBJS_BUILD_FLAGS            =
 endif
 LIBJS                            = $(LIBJS_BUILD_DIR)/libjs_static.a
+LIBJS_BUILD_STAMP                = $(LIBJS_BUILD_DIR)/build_stamp
+LIBJS_CONFIG                     = $(LIBJS_BUILD_DIR)/config.status
+LIBJS_CONFIG_STAMP               = $(LIBJS_BUILD_DIR)/config_stamp
 
-$(LIBJS): LIBJS_SRC
+
+.PHONY: all
+all: LIBJS_SRC $(LIBJS)
+
+$(LIBJS): $(LIBJS_BUILD_STAMP)
+
+$(LIBJS_BUILD_STAMP): $(LIBJS_CONFIG_STAMP)
 	@echo
 	@echo "Building Javascript library..."
 	@echo
-	cd $(LIBJS_BUILD_DIR) && ../configure $(LIBJS_CONFIG_FLAGS)
 	make -C $(LIBJS_BUILD_DIR) $(LIBJS_BUILD_FLAGS)
+	touch $@
+
+$(LIBJS_CONFIG_STAMP):
+	@echo
+	@echo "Configuring Javascript library..."
+	@echo
+	cd $(LIBJS_BUILD_DIR) && ../configure $(LIBJS_CONFIG_FLAGS)
+	touch $@
 
 .PHONY: LIBJS_SRC
 LIBJS_SRC:
-	echo "Updating Javascript sources..."
+	@echo
+	@echo "Updating Javascript sources..."
+	@echo
 	cd deps/Cocoa-deps/scripts && ./update-mozilla.sh
 	mkdir -p $(LIBJS_BUILD_DIR)
 
 .PHONY: clean
 clean:
 	-make -C $(LIBJS_BUILD_DIR) clean
+	-rm -f $(LIBJS_BUILD_STAMP)
+
+# This target also removes the configuration status, forcing
+# a reconfiguration. Use this after changing LIBJS_CONFIG_FLAGS
+.PHONY: distclean
+distclean:
+	-make -C $(LIBJS_BUILD_DIR) distclean
+	-rm -f $(LIBJS_CONFIG_STAMP)