cmake: Allow FFmpeg external usage and cleanup

Sets find_package(FFmpeg) to QUIET instead of REQUIRED. This allows
using the FFmpeg external in cases where there is no suitable installed
version of FFmpeg.

Also fixes a bug where multiple CMake configures causes FFmpeg_LIBRARIES
to concatenate on itself, producing cyclical dependencies. Unsets the
variable before building it in the foreach loop.

Fixes FFmpeg_INCLUDE_DIR not including the headers generated at run
time.