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:

Review URL: https://codereview.chromium.org/1234493002