Adjust Homebrew freetype detection logic

XQuartz ships an older freetype that still has a top-level "ft2build.h"
header file. Homebrew's freetype is newer and does not have this file,
it only has "freetype2/ft2build.h".

setup.py finds the header in XQuartz first, but Homebrew's compiler
wrappers intentionally strip out the XQuartz include paths during the
build unless the package depends on it explicitly.

We want to prefer Homebrew's freetype anyway, so if it's installed,
let's not even bother to search the XQuartz paths.

Fixes #653