Revert "Use clang on mac if XCode >= 4.5"
Revert "Use clang on mac if XCode >= 4.5"

We cannot build v8 after this change because clang reports a warning in
v8/src/parser.cc about an unused field (and we turn warnings into errors).

We can enable this change again after we update to a new v8 version (this seems
to be fixed in v3.17).

Review URL: https://codereview.chromium.org//14477015

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@21984 260f80e4-7a28-3924-810f-c04153c831b5

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