_XcodeVersion: return CLT version as appropriate

If Xcode isn't installed passing the CLT version is the next best bet.
This ports the CLT version function from Homebrew, and calls it if
xcodebuild fails to run.

BUG=code.google.com/p/gyp/issues/detail?id=292
Review URL: https://codereview.chromium.org/117553003/

Patch from mistydemeo@gmail.com!

Review URL: https://codereview.chromium.org/106293003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1826 78cadc50-ecff-11dd-a971-7dbc132099af