Revert "Revert "Add external projects only CFlags option""

This reverts commit a9ccbb7b87b9f7f30e38f1f79c1de064ffbcc392.

Test: m
Change-Id: I92d7b7707f73ef3af5824d3e8cb6da0465b5e07d
diff --git a/cc/config/clang.go b/cc/config/clang.go
index ba1cd3c..22e428f 100644
--- a/cc/config/clang.go
+++ b/cc/config/clang.go
@@ -182,6 +182,11 @@
 		// compatibility.
 		"-Wno-c++98-compat-extra-semi",
 	}, " "))
+
+	// Extra cflags for projects under external/ directory
+	pctx.StaticVariable("ClangExtraExternalCflags", strings.Join([]string{
+		// TODO(yikong): Move -Wno flags here
+	}, " "))
 }
 
 func ClangFilterUnknownCflags(cflags []string) []string {
diff --git a/cc/config/global.go b/cc/config/global.go
index 06f6f9a..c734c2e 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -170,6 +170,8 @@
 	pctx.StaticVariable("CommonClangGlobalCppflags",
 		strings.Join(append(ClangFilterUnknownCflags(commonGlobalCppflags), "${ClangExtraCppflags}"), " "))
 
+	pctx.StaticVariable("ClangExternalCflags", "${ClangExtraExternalCflags}")
+
 	// Everything in these lists is a crime against abstraction and dependency tracking.
 	// Do not add anything to this list.
 	pctx.PrefixedExistentPathsForSourcesVariable("CommonGlobalIncludes", "-I",