Tweak GYP to also link with XCode 7.

gyp/zlib.gyp

Index: gyp/zlib.gyp
diff --git a/gyp/zlib.gyp b/gyp/zlib.gyp
index 7ae62309d25f50939ebe924c940c94491fd9b5a8..1a79715f9eb2563a2efd3983f5b1a48e0ebbb7be 100644
--- a/gyp/zlib.gyp
+++ b/gyp/zlib.gyp
@@ -30,8 +30,8 @@
         'conditions': [
           [ 'skia_android_framework', { 'include_dirs': [ 'external/zlib' ] }],
           [ 'skia_os == "mac" or skia_os == "ios"', {
-              # XCode needs and explicit file path, not a logical name like -lz.
-              'link_settings': { 'libraries': [ '$(SDKROOT)/usr/lib/libz.dylib' ] },
+              # XCode needs a full library name, not -lz.
+              'link_settings': { 'libraries': [ 'libz.dylib' ] },
           }],
           [ 'skia_os not in ["mac", "ios", "win"]',{
               'link_settings': { 'libraries': [ '-lz' ] },