bsdiff: adopt and clean up

Bonus: now goes from broken to working! I'm pretty sure the last expression
didn't actually properly fix the (broken) installation step. Anyway,
it's easier to just build the programs directly than hacking the BSD
makefile with `sed` or whatever.

This should also fix the build of bsdiff for Darwin that I saw fail on
Hydra, due to a missing include (patch taken from Homebrew).