GUB for LilyPond 2.21

[Jonas Hahnfeld]

I think I just finished fighting GUB to produce working binaries for
the next unstable LilyPond 2.21. Please review the PR and merge:
https://github.com/gperciva/gub/pull/71
I propose to ditch FreeBSD because the binaries are not working anyway
and darwin-ppc/linux-ppc which is 32 bit PPC and not relevant anymore.

For mingw we need a few fixes. These are currently in branch
dev/hahnjo/cross-mingw and under review at 
https://sourceforge.net/p/testlilyissues/issues/5799/
Other than that, I think tooling should be ready for the next unstable
series!

Jonas

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