configure: error if requested feature unavailable
configure: error if requested feature unavailable

And fix fontconfig's help string; 7a9b40bfe0ec240a7e74b36e2e2f284d00b1023c
made fontconfig required by default, but it was ommitted to change the
help string back to "check" when the default behaviour changed again in
928f7a3c7e53daee99b3a27a50199192393e224f.

MrSmile/libass,astiob/libass,astiob/libass,ShiftMediaProject/libass,ShiftMediaProject/libass,MrSmile/libass,libass/libass,libass/libass