Update the GIT revision properly

src/Makefile.am

@@ -13,7 +13,7 @@ EXTRA_DIST = extras/COPYING modules/builtin.h.in \
 	libvlc.sym \
 	libvlccore.sym
 
-BUILT_SOURCES = modules/builtin.h misc/revision.c ../include/vlc_about.h
+BUILT_SOURCES = modules/builtin.h ../include/vlc_about.h
 CLEANFILES = $(BUILT_SOURCES)
 
 SUFFIXES = .pc.in .pc
@@ -468,13 +468,21 @@ SOURCES_libvlc_control = \
 # Stamp rules
 ###############################################################################
 
+misc/revision.c.tmp:
+	rm -f -- $@
+	echo "/* AUTOGENERATED FILE - DO NOT EDIT */" > $@
+	REVISION="$$(cd "$(top_srcdir)"; git describe 2>/dev/null || echo "$(VERSION)")"; \
+	echo "const char psz_vlc_changeset[] = \"$$REVISION\";" >> $@
+	diff $@ misc/revision.c >/dev/null 2>&1 || \
+		cat misc/revision.c.tmp > misc/revision.c
+
 misc/revision.c:
-	rm -f -- $@ $@.tmp
-	echo "/* AUTOGENERATED FILE - DO NOT EDIT */" > $@.tmp
-	REVISION="$$(cd "$(top_srcdir)"; git describe 2>/dev/null || echo exported)"; \
-	echo "const char psz_vlc_changeset[] = \"$$REVISION\";" >> $@.tmp
-	mv -f -- $@.tmp $@
+	$(MAKE) misc/revision.c.tmp
+	cat misc/revision.c.tmp > misc/revision.c
+
+BUILT_SOURCES += misc/revision.c.tmp misc/revision.c
 
+.PHONY: misc/revision.c.tmp
 
 ###############################################################################
 # Unit/regression test