Makefile.cmt: fix for progress reporting on Windows

Makefile.cmt

@@ -210,8 +210,11 @@ purge: clean $(foreach dep,$(wildcard $(dependency_dir)/*.pack.d),$(dep)_do_purg
 #purge: clean
 #	$(RM) $(dependency_dir)/*.pack.d $(packages:=/cmt/$(Makefile))
 
+build_flags = $(wildcard $(dependency_dir)/*.pack.build)
 remove_build_flags:
-	@$(RM) -r $(dependency_dir)/*.pack.build
+ifneq ($(build_flags),)
+	@$(RM) -r $(build_flags)
+endif
 
 # --- Add rules to build packages (e.g. "make MyHat/MyPackage")
 $(foreach pack,$(packages),$(eval $(pack): $(pack)_build))