Add MinGW and MSys support for libao

[jmid]

This PR adds MinGW and MSys support for libao. It is based on

• https://packages.msys2.org/base/mingw-w64-libao
• https://cygwin.com/packages/summary/mingw64-x86_64-libao.html
• https://cygwin.com/packages/summary/mingw64-i686-libao.html

The PR is modeled after #26072 by @dra27 who wrote (I quote): 😃

I'm very happy to be pinged to review them

Polite ping to @gridbugs who struggled to get this working https://www.gridbugs.org/sound-on-ocaml-on-windows/ and thus may be interested in testing it... 🤞

[gridbugs]

While working on my blog post about libao I used exactly this depext entry for cygwin and found that it didn't work properly. The mingw64-x86_64-* package seem to install to \root\usr\x86_64-w64-mingw32\sys-root\mingw\lib, and I found that pkgconf doesn't look in this directory when searching for libraries. I only managed to get this working on msys2 (using the exact same line you have below).

[jmid]

Did you invoke pkgconf with the --personality=x86_64-w64-mingw32 flag like the tests in conf-mingw-w64-ao-x86_64.1/opam and conf-ao.1/opam do?

[gridbugs]

Ohh interesting, no I did not. That might be what I was doing wrong. Let me see if that works for me!

[gridbugs]

I've tested this on my x86_64 windows machine with opam configured to use msys2 and I can build the ao package after pinning the conf-ao package from this branch. While you were developing this which windows package manager did you use for testing? I'm curious about whether you've found a way to make the cygwin lib-ao package work as I was unable to when I tried a few months ago. I might set up an opam environment on my windows machine with cygwin and see if it works for me.

[jmid]

I've tested this on my x86_64 windows machine with opam configured to use msys2 and I can build the ao package after pinning the conf-ao package from this branch.

Excellent! 🎉

While you were developing this which windows package manager did you use for testing?

I've just used the CI workflow, which at the moment only tests MinGW (installed via Cygwin).
I have hacked an extension of it that also tests on MSys, where it also held up nicely.

[gridbugs]

I just set up a cygwin-based opam environment and confirmed that I can also build the ao package there after pinning the relevant packages from this PR. TIL about pkgconf --personality. Nice work @jmid!

[mseri]

I am going to merge. This is a strict improvement over the previous situation. Thanks!