Add external projects only CFlags option

Every time the toolchain adds a new warning, we had to suppress the
warning in every external projects that violates it, or disable it
globally -- which we end up doing most of the time since the first
option is way too time consuming.

Add a separate cflags option for external projects and move existing
-Wno-* flags there so that we can enforce better code quality for
internal projects.

TODO: Move more -Wno-* flags to ClangExtraExternalCflags

Test: m checkbuild
Change-Id: If3dee491616a1e7ba6223c2f522d100e10c5ee76

cc/config/clang.go

@@ -173,14 +173,17 @@ func init() {
 		// this new warning are fixed.
 		"-Wno-null-pointer-arithmetic",
 
+		// Disable c++98-specific warning since Android is not concerned with C++98
+		// compatibility.
+		"-Wno-c++98-compat-extra-semi",
+	}, " "))
+
+	// Extra cflags for projects under external/ directory
+	pctx.StaticVariable("ClangExtraExternalCflags", strings.Join([]string{
 		// http://b/72330874 Disable -Wenum-compare until the instances detected by this new
 		// warning are fixed.
 		"-Wno-enum-compare",
 		"-Wno-enum-compare-switch",
-
-		// Disable c++98-specific warning since Android is not concerned with C++98
-		// compatibility.
-		"-Wno-c++98-compat-extra-semi",
 	}, " "))
 }