pull gflags.gyp changes from webrtc

gflags has a build warning in the chromium version.

../../third_party/gflags/src/gflags.cc:281:53: error: unused typedef 'true_false_equal' [-Werror,-Wunused-local-typedef]
    COMPILE_ASSERT(sizeof(kTrue) == sizeof(kFalse), true_false_equal);
                                                    ^
1 error generated.
[8/231] CXX obj/chromium/src/testing/gtest/src/gtest.gtest.o

Its been reported and fixed upstream
https://github.com/gflags/gflags/issues/125

But not pulled into chromium.  Last update to flags was in 2013.
This change disables the warning.

R=harryjin@google.com, kjellander@chromium.org, jkellander@chromium.org
BUG=libyuv:536, webrtc:5397, webrtc:5398, webrtc:5399

Review URL: https://codereview.chromium.org/1581123002 .

third_party/gflags/BUILD.gn

@@ -28,6 +28,13 @@ config("gflags_config") {
     "GFLAGS_DLL_DECLARE_FLAG=",
     "GFLAGS_DLL_DEFINE_FLAG=",
   ]
+
+  # GN orders flags on a target before flags from configs. The default config
+  # adds -Wall, and this flag have to be after -Wall -- so they need to
+  # come from a config and can't be on the target directly.
+  if (is_clang) {
+    cflags = [ "-Wno-unused-local-typedef" ]
+  }
 }
 
 source_set("gflags") {
@@ -45,9 +52,7 @@ source_set("gflags") {
     ]
   }
 
-  include_dirs = [
-    "$gflags_gen_arch_root/include/private",  # For config.h
-  ]
+  include_dirs = [ "$gflags_gen_arch_root/include/private" ]  # For config.h
 
   public_configs = [ ":gflags_config" ]
 
@@ -60,3 +65,4 @@ source_set("gflags") {
     configs -= [ "//build/config/clang:extra_warnings" ]
   }
 }
+

third_party/gflags/gflags.gyp

@@ -76,8 +76,10 @@
           },
         }],
         ['clang==1', {
+          'cflags': ['-Wno-unused-local-typedef',],
           'cflags!': ['-Wheader-hygiene',],
           'xcode_settings': {
+            'WARNING_CFLAGS': ['-Wno-unused-local-typedef',],
             'WARNING_CFLAGS!': ['-Wheader-hygiene',],
           },
         }],
@@ -85,3 +87,4 @@
     },
   ],
 }
+