Meson: Support Windows builds

Meson: Support Windows builds

Add the necessary machinery into the Meson definition files so that we
can build for Windows.

Since we don't have Wayland or X support for our use case here, disable
them once we know that we are building for Windows, as they are
(otherwise) enabled by default, and enable the items that need to be
built for Windows builds.

Exclude gtk4-launch from Windows builds as that is something that
is not supported on Windows.

As we won't have gio-unix on Windows, and PangoFT2 is optional, don't use
fallbacks for them when we are on Windows (but do use fallbacks for
gio-win32, as it will be used).

Also, clean up meson.build a bit as we can just force-include
msvc_recommended_pragmas.h from GLib since we depend on GLib, and so we
can handle these warnings from msvc_recommended_pragmas.h instead.

https://bugzilla.gnome.org/show_bug.cgi?id=785210