"clang: warning: not using the clang compiler for C++ inputs" due to hard-coding of /usr/bin/clang in WebCore.gyp
https://bugs.webkit.org/show_bug.cgi?id=108089

Reviewed by Ojan Vafai.

Use gcc since older versions of clang (with Xcode 3.2) warn that they are going to use gcc anyway.
These warnings are showing up on the main Chromium waterfall too:
http://build.chromium.org/p/chromium/builders/Mac/builds/19113/steps/compile/logs/stdio

No new tests, this is a build change.

* WebCore.gyp/WebCore.gyp:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141007 268f45cc-cd09-0410-ab3c-d52691b4dbfc