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

This reverts commit a9ccbb7b87.

Test: m
Change-Id: I92d7b7707f73ef3af5824d3e8cb6da0465b5e07d

cc/config/clang.go

@@ -182,6 +182,11 @@ func init() {
 		// 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 {