configure: abort if FFmpeg libraries are not found

configure: abort if FFmpeg libraries are not found

Abort if FFmpeg libraries are not found unless --disable-ffmpeg was
explicitly specified. Note that even if you disable FFmpeg, libavutil
and libswscale are still required and you'll need to add linker flags
for them manually; I didn't try to clean up that case (few people
will want to do that).

Also fix the fallback test used if pkg-config doesn't find the
libraries: fix wrong header name and drop forced -lavcore which isn't
present in all FFmpeg versions. This shouldn't make a difference for
normal use since the pkg-config test should succeed if the libraries
are available.