Support a custom suffix for FFmpeg binaries

Just like MLT's --avformat-suffix, FFMPEG_SUFFIX allows for
definition of custom suffix to append to ffmpeg and ffplay binaries.
This is mostly useful for packagers willing to set a default version
for FFmpeg dependency, as users can already configure ffmpeg and
ffpaly paths via settings.

Conflicts:
	src/monitor/recmonitor.cpp

Conflicts:
	src/recmonitor.cpp