OS X: Resolve compiler in /usr/bin to that reported by Xcode xcrun

The compiler in the PATH on mac is a stub for a different delegate
depending on the environment.  Rather than requiring xcode-select to
change the used Xcode globally, users should be able to choose the
compiler per-session.  That is possible with the DEVELOPER_DIR
environment variable.

However, the environment can change between running CMake and invoking
the build.  In such cases, CMake prefers to record the relevant paths
from the environment and use them when invoking the build.  That is not
currently done for the compilers on APPLE, so the compiler used is not
the one reported when running cmake:

 $ DEVELOPER_DIR=/Applications/Xcode2.app/Contents/Developer/ cc --version
 Apple LLVM version 6.0 (clang-600.0.51) (based on LLVM 3.5svn)
 Target: x86_64-apple-darwin13.4.0
 Thread model: posix

 $ DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer/ cc --version
 Apple LLVM version 5.1 (clang-503.0.38) (based on LLVM 3.4svn)
 Target: x86_64-apple-darwin13.4.0
 Thread model: posix

Update that now by querying Xcode for the correct compiler path if
the compiler located by ordinary means is located in /usr/bin.

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