This is mainly a note for myself.

My problem was that I had ffmpeg installed from MacPorts, but pkg-config from Fink. Thus pkg-config does not see the .pc files installed from macports. Thus configure cannot find the ffmpeg libraries even though I set the include path... The solution is to unpack the package into a directory and issue the command

$ env PKG_CONFIG_PATH=/opt/local/lib/pkgconfig/ cabal install

(rant: I really *really* hate the opensource "ecosystem" with the milliard stupid dependencies from the hell and stupid build systems and broken repositories, it is a lesser miracle when *any* program builds from source, and there is *never* a binary build online)