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).

Committed: https://code.google.com/p/dart/source/detail?r=21984

[hausner, 2013-04-24 21:05:03]

lgtm

[kustermann, 2013-04-24 21:05:43]

Committed patchset #1 manually as r21984 (presubmit successful).