Add new alternate repository for mxe

[kevingranade]

Summary

SUMMARY: None

Purpose of change

The mxe project has put up a new repository where their packages can be fetched, which unblocks us to re-enable mxe-based builds.

Describe the solution

Adjust the mxe build targets to reference the new repository.

[Firestorm01X2]

Just don't forget about this:
#23446

#23446 (comment)

I've managed to resolve issue for me.
I've just grab specific (older) version of MXE:
git checkout git rev-list -n 1 --before="2017-09-24 12:00" master``

This version built non bugged version of the game. Graphical isses with tilesets gone away.

But. Watch out! It is probably means that if you will update MXE on build server- then you will get that kind of issues since build server uses old version of MXE.

But who knows, maybe that but is not present in newest MXE. Otherwise

git checkout git rev-list -n 1 --before="2017-09-24 12:00" master``

Should give version without tileset graphical artifacts aka black lines.

[kevingranade]

I'm planning on merging this once the current build completes successfully, but if someone beats me to it, please squash these commits, there's no need to have all these guesses in the build history.