libav-config: support pre-tool-rename versions
Various executable tools were earlier renamed in Libav (ffplay->avplay
etc) and the corresponding configure --disable-xyz switches were
renamed accordingly. Make libav-config check which tool names exist in
libav/ and add --disable switches according to that. This allows
checking out older Libav versions (for testing) or FFmpeg in the
libav/ dir and compiling against those; previously the --disable-avplay
switch would be used unconditionally, and would cause a failure if the
library version didn't recognize that name.