[zcash/zcash] Windows cross-compile support (#3172)

Ubuntu 18.x….. Ubuntu 16.x, even when it built past versions, has a buggy version of the mingw pthreads library, so even if you got the current release to build on it, you’d have a zcashd.exe that pegs a whole CPU core even when synced and idling (its got a context switch bug).

So right now Ubuntu 18 is the way to go