MinGW cross-compilation support

[ludo@gnu.org (Ludovic Courtès)]

Hello Guix!

After all this time I’m happy to report that I’ve finally merged MinGW
cross-compilation support, woohoo!

So we should now be able to do:

  guix build --target=i686-w64-mingw32 guile

to cross-compile Guile to MinGW.

Hydra will build the cross-compilation toolchain and some example
packages.

I didn’t merge the cross-libtool patch and this one:

Jan Nieuwenhuizen <janneke@gnu.org> skribis:

> * gnu/packages/ncurses.scm (ncurses)[MINGW]: Build with libtool, as
> recommended; enables dlopen'ing.

I’m not sure what this means (and the explanation should be as a comment
in the code).  Could you explain a bit more?

The idea of Libtool is normally to bundle it in the package tarball.
That ncurses can optionally take an externally-provided Libtool is weird
and I’d rather avoid relying on that if possible.


Adding a new cross-compilation target is a commitment.  So I hope you
and others will make sure it remains functional and useful!

I also think that together with Manolis and everyone else who’s played
with cross-compilation, we must clean up the mess that this has become.
;-)  Namely, we must more clearly separate target-specific things and
also separate build-side from host-side code (in cross-base.scm).

Suggestions and hacks welcome!

Thanks,
Ludo’.