CI: Add a x86 MinGW-w64 cross build

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 archives (not git, as we don't really want
this to randomly break on changes to master in those dependencies).

Don't build with GLX enabled, so I don't have to deal with
cross-building mesa and it's dependencies, for the moment.
5 jobs for gitlab-ci-cross-mingw in 5 minutes and 56 seconds
latest
Status Job ID Name Coverage
  Docker Image
passed #435673
debian-testing

00:00:19

 
  Build And Test
passed #435674
autotools-build-and-test

00:05:36

passed #435675
meson-build-and-test

00:02:06

passed #435676
mingw-cross-build

00:01:36

 
  External
passed continuous-integration/appveyor/branch