gyp: clean up warnings with clang-3.8

gyp/zlib.gyp

Index: gyp/zlib.gyp
diff --git a/gyp/zlib.gyp b/gyp/zlib.gyp
index 43cf6abfd23c5776994a03852249a6aabe18e77f..6263c6697b9d2381d4d0cd6cfed4ed29437daa64 100644
--- a/gyp/zlib.gyp
+++ b/gyp/zlib.gyp
@@ -19,8 +19,14 @@
           },
         }, {
           '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',