atk: Only pass -Wno-unused-local-typedef with clang.

GCC's analogous command-line argument is -Wno-unused-local-typedefs, with an
's' at the end, and //build/config/compiler/BUILD.gn already passes it to
the compiler unconditionally when clang is not being used.

This CL prevents warnings in the GCC build that look like this:

    cc1plus: warning: unrecognized command line option ‘-Wno-unused-local-typedef’

Change-Id: Icec95a6d1cf3a4bcd4f3a0acb7f002850aa9f7af
Reviewed-on: https://chromium-review.googlesource.com/980942Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#546113}

build/config/linux/atk/BUILD.gn

@@ -39,12 +39,14 @@ config("atk") {
   configs = [ ":atk_base" ]
 
   cflags = [
-    # glib uses the pre-c++11 typedef-as-static_assert hack.
-    "-Wno-unused-local-typedef",
-
     # G_DEFINE_TYPE automatically generates a *get_instance_private
     # inline function after glib 2.37. That's unused. Prevent to
     # complain about it.
     "-Wno-unused-function",
   ]
+
+  if (is_clang) {
+    # glib uses the pre-c++11 typedef-as-static_assert hack.
+    cflags += [ "-Wno-unused-local-typedef" ]
+  }
 }