AviSynth+ 3.7.1a

Hotfix release

Needed for version checking with external projects when pkg-config cannot be used.

Luckily, we already have version.h for this, so just copy it to the avs subdir on install. This also required copying arch.h as well.

As this is virtually 1:1 identical with 3.7.1 apart from the Histogram Audiolevels fix and some docs changes, there's really no need to run off new builds for Windows or Mac. Distribution managers for Linux, et al., will want to use 3.7.1a.