nix mingw issues for x86/amd64

Only lightly tested, as only output binary copied to target system, no units ran.

Sanity-checked as follows % { echo boardsize 19; while :; do echo genmove b; echo genmove w; done; } | nice -n 20 ./pachi-x86-64.exe -t 60 threads=8

note, despite messing with max tree size and time argument, couldn't get it to to consume more than 1.1 gigs worth of ram.