Specified drm location for FreeBSD + fixed anonymous union

Under FreeBSD there's no <libdrm/drm.h> but the other possibility mentioned in compton.h - <drm/drm.h> . I used __FreeBSD__ define to check the system and specify another header location. There was also a strange mistake with an anonymous union which I commented out to make it work.
Leaving those matters aside compton is a great app, keep up the good work!