Incorrect CLT warning

When "Command Line Tools (OS X Mountain Lion) for Xcode - January 2013" (xcode46cltools_10_86938131a.dmg) is installed `brew doctor` warns that there is a more recent version available.

I assume this is due to a new output format of `/usr/bin/clang -v`, so I updated the regular expression to correctly identify the installed `clang` version.

After doing so and running `brew doctor` again, Homebrew warned that my "compilers are different from the standard versions for your Xcode" even if I don't have Xcode installed. Since Xcode isn't a requirement anymore, I changed the compiler check to only happen when Xcode is installed.

`brew doctor` and `brew --config` output: https://gist.github.com/1561fd3215b6cbbde0aa (with `/usr/bin/clang -v` bonus)