Add AvxSynth plugin support to the non-autotools build system.

This makes it possible to update avxffms2 without building all of
AvxSynth, as well as dual-purpose AvxSynth/VapourSynth plugins,
like the existing AviSynth/VapourSynth capability.
Cosmetic changes to configure's options.

--enable-avisynth and --enable-avxsynth are added. The short named
options (--enable-vs has been added to these) won't be removed,
but not mentioned in the help.

The configure report now also explicitly says 'avisynth' and
'avxsynth'.