Only cross-build for x86 MinGW-w64, as x64 MinGW-w64 will probably
require fixing LLP64 issues.
I don't know any source for pre-built cross-dependencies of the X
server, so build these from git.
Don't build with GLX enabled, so I don't have to deal with
cross-building mesa and it's dependencies, for the moment.
Jon Turney