gyp: clean up warnings with clang-3.8

gyp/zlib.gyp

 # Copyright 2014 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'skia_warnings_as_errors': 0,
   },
   'targets': [
   {
       'target_name': 'zlib',
       'conditions': [
         [ 'skia_android_framework', {
           'type': 'none',
           'direct_dependent_settings': {
               'include_dirs': [ 'external/zlib' ],
               'link_settings': { 'libraries': [ '-lz' ] },
           },
         }, {
           'type': 'static_library',
-          'cflags': [ '-Wno-unused-value' ],
-          'xcode_settings': { 'WARNING_CFLAGS': [ '-Wno-unused-value', ], },
+          'variables': {
+            'skia_zlib_flags' : [
+              '-Wno-unused-value',
+              '-Wno-shift-negative-value',

[bungeman-skia, 2016/05/23 20:53:10]

It would be nice to comment here that this was already fixed in the zlib
development branch by "Avoid shifts of negative values inflateMark().". It would
also be nice if Chromium hadn't foobared their zlib fork (it isn't really a
mirror). Alas, probably not even worth the comment.

+            ],
+          },
+          'cflags': [ '<@(skia_zlib_flags)' ],
+          'xcode_settings': { 'WARNING_CFLAGS': [ '<@(skia_zlib_flags)' ], },
           'sources': [
             '../third_party/externals/zlib/adler32.c',
             '../third_party/externals/zlib/compress.c',
             '../third_party/externals/zlib/crc32.c',
             '../third_party/externals/zlib/deflate.c',
             '../third_party/externals/zlib/gzclose.c',
             '../third_party/externals/zlib/gzlib.c',
             '../third_party/externals/zlib/gzread.c',
             '../third_party/externals/zlib/gzwrite.c',
             '../third_party/externals/zlib/infback.c',
@@ skipping 13 matching lines @@
               '../third_party/externals/zlib',
             ],
           },
           'defines': [
             '_CRT_NONSTDC_NO_DEPRECATE',
           ],
         }],
       ],
   }],
 }