Fix Meson configuration under Meson 1.9.2

[bhennion]

Configuration fails under Meson 1.9.2 when the list gl_reqs is empty.

This is fixed by passing the actual list of strings to pkg.generate(), instead of the aggregated string.

[bhennion]

More info on that:

1. This PR passes the requires_private argument as a list instead of a single string. This corresponds to Meson's documentation: see https://mesonbuild.com/Pkgconfig-module.html:

   • requires: list of strings, pkgconfig-dependencies or libraries that pkgconfig.generate() was used on to put in the Requires field. Since 1.9.0 internal dependencies are supported if pkgconfig.generate() was used on the underlying library.
   • requires_private: the same as requires but for the Requires.private field

2. The changes in Meson (first affected version: 1.9.2) that made libepoxy's script raise an error are from pkgconfig: Handle malformatted string dependencies mesonbuild/meson#13951

3. This error will become a warning in Meson 1.10, thanks to pkgconfig: Improve handling of empty string deps mesonbuild/meson#15116
   Either way, better to pass the dependencies as a list rather than an aggregated string. See 1. above.

[bhennion]

It seems your MSys CI tests are out of date

Installing additional packages through pacman...
  C:\Windows\system32\cmd.exe /D /S /C D:\a\_temp\setup-msys2\msys2.cmd -c "'pacman' '--noconfirm' '-S' '--needed' '--overwrite' '*' 'base-devel' 'git' 'mingw-w64-x86_64-meson' 'mingw-w64-x86_64-ninja' 'mingw-w64-x86_64-pkg-config' 'mingw-w64-x86_64-python3' 'mingw-w64-x86_64-python3-pip' 'mingw-w64-x86_64-toolchain'"
  error: target not found: mingw-w64-x86_64-python3-pip

[rossburton]

FWIW, it's possible that there will be a meson 1.9.3 with the missing backport shortly, although fixing libepoxy to not warn is still the right fix.