Adjust version detection to rely on AVISYNTH_INTERFACE_VERSION.

Even if AvxSynth's version numbers are higher than AviSynth's,
the interface versioning is the same.  AvxSynth is still at 3,
like the version it was based on (2.5.8).

Also slightly adjusts the log messaging so that it reports
'avisynth' or 'avxsynth' based on which one is actually there.
Prompted by the fact that AvxSynth now reports its '4.0' version
numbers.