Meson: Support Visual Studio builds

[fanc999]

Hi,

This attempts to add support to the Meson build files to build with Visual Studio, by:

• Avoiding configuration errors since Lua may have been found via CMake on Visual Studio-like builds instead of pkg-config by only trying to retrieve Lua pkg-config variables if it is really found via pkg-config.
• Force-include msvc_recommended_pragmas.h on Visual Studio builds, so that we can silence compiler warnings that we really don't need to bother about and make the ones that we should really worry about glaring to us.
• Don't prefix DLL names with lib in core.c for Visual Studio-like builds, since the Meson build files for Cairo do not prepend such prefixes, which is where this comes to play.
• Fix building regress.dll by making sure symbols are exported, so that its introspection files can be properly built.
• Skip running the regress test for now on Windows since dbus-run-session does not currently exist on Windows.
• Fix running the (other) tests(s) on Windows by setting the envvars that we have set for the regress tests as well.

With blessings, thank you!

[psychon]

Thanks!

I had some minor comments when looking at the commits, but... yeah, dunno. It's windows, I have no clue, let's see what will happen. You provided nice, small commits with good commit messages, so hopefully whoever doesn't like this will "see what is going on".