* the libdrm is required but libva is not, so move out to prevent build errors
* put warnings when `vaapi` (libva-dev) and `vdpau` (libvdpau-dev) is not installed
* "correct" fix for inclusion of libdrm on Solaris (please, kill me baby)
* place "ffmpeg: " at begin of message
* also added on `docs` for better compatibility
Signed-off-by: Caio Oliveira <caiooliveirafarias0@gmail.com>