This got a bit messy as a change, but it's basically all that topic. I
had to go squint at python3-libraries to figure out the modern
incantation for getting python3 to build under the fuzzer, but victory
is mine and this is probably substantially cleaner.
This will clean up the Makefile on the hg side considerably, and will
also fix the coverage build. Fixes#2076.
Bonus: I did some extra work in build.sh so that incremental rebuilds
of the fuzzers won't needlessly recompile $OUT/sanpy, shortening
development time on new fuzzers and avoiding issues caused by a $OUT
that was built for a different fuzz configuration.
devtty1er