* Add an FFMPEG compile check from TheFluff, thanks! (modified)
* Add a libpostproc check from Myrsloik, thanks!. (reqired by ffms)
* Rework how ffms / FFMPEG providers are enabled/disabled.
* Fix PulseAudio check to use pulseaudio_required_version.
Originally committed to SVN as r2705.
from around r2140. An artifact from the very first commit of the new build
system was wiping CPPFLAGS, which removed -g if --enable-debug was used. I
didn't notice it locally as I was setting the flag differently. I wonder how
it went on this long without anyone noticing, next: useful crashdumps for all.
Originally committed to SVN as r2599.
