Makefile: Follow r118918, to add configuration "ENABLE_DOCS".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@119025 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/Makefile b/Makefile
index f871c25..ec5aa87 100644
--- a/Makefile
+++ b/Makefile
@@ -37,6 +37,10 @@
 # Include LLVM common makefile.
 include $(LEVEL)/Makefile.common
 
+ifneq ($(ENABLE_DOCS),1)
+  DIRS := $(filter-out docs, $(DIRS))
+endif
+
 # Set common Clang build flags.
 CPP.Flags += -I$(PROJ_SRC_DIR)/$(CLANG_LEVEL)/include -I$(PROJ_OBJ_DIR)/$(CLANG_LEVEL)/include
 ifdef CLANG_VENDOR