Don't use AC_CONFIG_SUBDIRS to call the FFmpeg configure script, as it
complains about all the unknown parameters autoconf gives it, and fiddling
with ac_configure_args makes maintainer-mode call our real configure script
with a bunch of bogus arguments.

Instead, use AC_CONFIG_COMMANDS to call the FFmpeg configure script
ourselves.

Remove autogen.sh code that modifies the FFmpeg configure script, as it's
not needed now that we only pass it arguments it understands, and move
the detection of flags like --disable-ffmpeg into the configure script,
otherwise they never get passed to FFmpeg if we call configure ourselves,
such as from a tarball.