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

This reverts commit a9ccbb7b87b9f7f30e38f1f79c1de064ffbcc392.

Test: m
Change-Id: I92d7b7707f73ef3af5824d3e8cb6da0465b5e07d
diff --git a/cc/compiler.go b/cc/compiler.go
index 8f119cf..10cec8c 100644
--- a/cc/compiler.go
+++ b/cc/compiler.go
@@ -392,6 +392,12 @@
 			fmt.Sprintf("${config.%sGlobalCflags}", hod))
 	}
 
+	if flags.Clang {
+		if strings.HasPrefix(android.PathForModuleSrc(ctx).String(), "external/") {
+			flags.GlobalFlags = append([]string{"${config.ClangExternalCflags}"}, flags.GlobalFlags...)
+		}
+	}
+
 	if ctx.Device() {
 		if Bool(compiler.Properties.Rtti) {
 			flags.CppFlags = append(flags.CppFlags, "-frtti")