[PATCH]: check for src/ subdirectories on clean

[Kel Modderman]

If src/wps/ is to be pruned from the release tarball by build_release, than
"make clean" should not fail.

Check for existence of each directory in src/ in clean target.

Signed-off-by: Kel Modderman <kel at otaku42.de>
---
--- a/src/Makefile
+++ b/src/Makefile
@@ -4,5 +4,5 @@
 	@echo Nothing to be made.
 
 clean:
-	for d in $(SUBDIRS); do $(MAKE) -C $$d clean; done
+	for d in $(SUBDIRS); do [ -d $$d ] && $(MAKE) -C $$d clean; done
 	rm -f *~
---