Hide Homebrew related variables in CMake GUI.

- Results of find_program() are by default visible in the CMake GUI
  (cache), but the user has no reason to care in general about the
  location of the Homebrew executable detected.
- Note it is still available in the GUI, but only via toggling to the
  advanced display.

Change-Id: Ifb0e54c00d350cbfb3b2717d889570cab61cce54