add pkgconfig to homebrew install instruction

build will not find freetype headers without it.

Alternate fix would be to search for freetype in the homebrew location (`/usr/local/Cellar/freetype/VERSION/`), or add the right location to the include path (`/usr/local/include/freetype2`).
Merge pull request #1936 from minrk/macpkgconfig

add pkgconfig to homebrew install instruction