vslsmas: Remove dummy definition of avresample_free().

It is unneeded anymore since common/audio_outout.o containing
avresample_free() is not linked.
lsmashsource: Add 'Y8' and 'RGB32' into 'format'.
Add install and uninstall recipes to VapourSynth plugin makefile
Clean up the Git-based version info generation a bit.

'git describe' can be fragile about what it reports, and using
rev-list's --count option removes the need to use wc, which also
means the use of sed to clean up wc's output on OS X is no longer
necessary either.