Fix thinko and enable clang build on mingw again (hopefully)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@98492 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/Makefile b/tools/Makefile
index 5a9c674..bfd5ad1 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -13,8 +13,7 @@
 include $(LEVEL)/Makefile.config
 
 ifeq ($(OS), $(filter $(OS), Cygwin MingW))
-DIRS := $(filter $(DIRS), CIndex)
-DIRS := $(filter $(DIRS), c-index-test)
+DIRS := $(filter-out CIndex c-index-test, $(DIRS))
 endif
 
 include $(LEVEL)/Makefile.common