Tweak GYP to also link with XCode 7.

Tweak GYP to also link with XCode 7.

Using the XCode 7 beta, the file in the GYP doesn't exist, instead we get

/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/lib/libz.tbd

which is a text file describing libz and pointing to /usr/lib/libz.1.dylib.

There's a weird easy fix, which is that GYP looks for things in libraries like 'libz.dylib' and pattern match translates that to '-lz' on the command line.  (Infuriatingly, a literal '-lz' is interpreted as a file path...)

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/24d8249638706acc28b32959aa7b093cf356f38a

[mtklein_C, 2015-07-10 02:36:47]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-10 02:37:03]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1234493002/1

[commit-bot: I haz the power, 2015-07-10 02:38:38]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-10 02:38:39]

Dry run: Try jobs failed on following builders:
  Build-Mac10.8-Clang-x86_64-Release-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Mac10.8-Clang-...)

[mtklein_C, 2015-07-10 12:34:01]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-10 12:34:05]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1234493002/20001

[mtklein_C, 2015-07-10 12:40:10]

mtklein@chromium.org changed reviewers:
+ reed@google.com

[commit-bot: I haz the power, 2015-07-10 12:41:08]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-10 12:41:08]

Dry run: This issue passed the CQ dry run.

[mtklein, 2015-07-13 14:48:06]

mtklein@google.com changed reviewers:
+ mtklein@google.com

[mtklein, 2015-07-13 14:48:06]

The CQ bit was checked by mtklein@google.com

[mtklein, 2015-07-13 14:48:06]

lgtm

[commit-bot: I haz the power, 2015-07-13 14:48:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1234493002/20001

[commit-bot: I haz the power, 2015-07-13 15:13:06]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/24d8249638706acc28b32959aa7b093cf356f38a