configure.ac: Rework optional dependency checking code

Now configure exists with error in case user has requested to enable certain feature but its dependency hasn't been found.

Also, use AS_HELP_STRING and ensure proper m4 quoting around it.


I did it because I was always compiling (I use Gentoo Linux) the plugins with --enable-ffaudio, and now when I replaced ffmpeg with libav in my system, it began to silently fail. If there was an error by configure time, I wouldn't have to wonder why audacious stopped playing wma files. I think my patch will save trouble for people in the future, if applied.