Unable to pick up correct include paths on MinGW

[LordAro]

I've been on a bit of a journey today trying to get XML::LibXML to build for Windows perl on a UCRT64 (MSYS2 subsystem). It's proving quite difficult.

The core of the issue is that Alien::LibXML2 is unable to find the existing libxml install so the -I/ucrt64/include/libxml paths passed to gcc don't work (because it has no idea what to do with them). I've spent quite a lot of time (and debug prints) working out why this is. This is quite similar to a number of issues

The alienfile defines several probe methods:

• vcpkg - not installed, not used
• xml2-config - this doesn't work because it's a shell script, so calling via system on Windows doesn't find anything even though the directory is on the PATH. Not sure if there's a way of calling via the MSYS-bash shell, but it'd probably have to be explicitly listed (sh -c 'xml2config...')
• pkgconfig - main one I've focused on, see below
• Probe::CBuilder - try_flags only lists Linux paths, never going to work

By default, the Alien::Build::Plugin::PkgConfig module uses the PP submodule, which looks at the PkgConfig library. As the libxml-2.0.pc file specifies prefix=/ucrt64, this results in the same incorrect path. I've left a comment on PerlPkgConfig/perl-PkgConfig#27 which mentions this

It should be noted at this point that actual pkg-config works fine, as it has an implicit --define-path which does path rewriting into the Windows path. Which leads into the CommandLine submodule, which does call the actual pkg-config, ...but doesn't work as it explicitly calls pkg-config with --dont-define-path when $meta->prop->{platform}->{system_type} eq 'windows-mingw' for whatever reason and can't be disabled. Sigh.

The only remaining option is PkgConfig::LibPkgConf which isn't a listed dependency and isn't installed, so I've not tested it (yet?)

I'm not sure if this is just "Alien::Build's issue to fix" or elsewhere, or if there's some other workaround possible, or if the alienfile should "try harder" to find the xml library, but I just thought I'd write it down and I'm all ears if there's some other workaround I've missed. I'm trying to install in an automated script with cpanm XML::LibXML so while modifying the Makefile.PL or other sources I guess are possible, it's not ideal...

(oh, and setting ALIEN_INSTALL_TYPE=share results in a Mozilla::CA module not found error for some reason?)