Makefile: don't run "rm" without any files

When COMPUTE_HEADER_DEPENDENCIES is set to "auto" and the compiler
does not support it, $(dep_dirs) becomes empty.  "make clean" runs
"rm -rf $(dep_dirs)", which can fail in such a case.

Signed-off-by: Matt Kraai <matt.kraai@amo.abbott.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2414,8 +2414,7 @@ clean: profile-clean
 		builtin/*.o $(LIB_FILE) $(XDIFF_LIB) $(VCSSVN_LIB)
 	$(RM) $(ALL_PROGRAMS) $(SCRIPT_LIB) $(BUILT_INS) git$X
 	$(RM) $(TEST_PROGRAMS)
-	$(RM) -r bin-wrappers
-	$(RM) -r $(dep_dirs)
+	$(RM) -r bin-wrappers $(dep_dirs)
 	$(RM) -r po/build/
 	$(RM) *.spec *.pyc *.pyo */*.pyc */*.pyo common-cmds.h $(ETAGS_TARGET) tags cscope*
 	$(RM) -r $(GIT_TARNAME) .doc-tmp-dir