Makefile: Refuse to build release from unclean repo

Root cause of an error during the 0.7.11 release

Makefile

@@ -208,6 +208,17 @@ $(PKG):
 		--out $(PKG)
 
 $(TARBALL): node out/doc
+	@if [ "$(shell git status --porcelain | egrep -v '^\?\? ')" = "" ]; then \
+		exit 0 ; \
+	else \
+	  echo "" >&2 ; \
+		echo "The git repository is not clean." >&2 ; \
+		echo "Please commit changes before building release tarball." >&2 ; \
+		echo "" >&2 ; \
+		git status --porcelain | egrep -v '^\?\?' >&2 ; \
+		echo "" >&2 ; \
+		exit 1 ; \
+	fi
 	@if [ $(shell ./node --version) = "$(VERSION)" ]; then \
 		exit 0; \
 	else \