Add support for fetching and building fribidi
libass-config: override FRIBIDI_CFLAGS

For some reason, seemingly, the locally installed fribidi pkg-config
info causes libass ./autogen.sh to add
"-I$(pwd)/build_libs/include/fribidi" instead of
"-I$(pwd)/build_libs/include". Accommodate for that by overriding
FRIBIDI_CFLAGS in scripts/libass-config.