Use pkgconfig to get libcommon include path

On some systems ogg, vorbis, and portaudio headers are not in the
default include path.  Use pkgconfig to add the include paths.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Add portmidi build paths for homebrew on Mac

The homebrew (http://brew.sh/) package manager should be used for
building on Mac.  Since portmidi does not ship a pkgconfig file we have
to hard-code the build paths.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>