This will attempt to include a PoW library (.so on Unix, .dll on
windows) to do PoW. This is done in a safe way and clean fallback to
normal PoW. The code for the library will be in another commit. The code
is take from https://github.com/bm-robertwhite/bitmessage-powfaster,
with minor modifications.
This patch also includes code to make GPU PoW have a safer fallback.