Autodetect Homebrew install prefix on OSX.

- Call through to Homebrew on OSX to determine it's install root
  in case it is not /usr/local and add the result to the CMake
  prefix path that is searched for all find_path/library calls.
- This should allow Ceres to compile even when Homebrew is installed
  in a non-standard location.

Change-Id: I230a5e12aef54617567bdfd20c4fd45c5a04a8bf