Make it a private static library to be used in commming commits.
Contrary to upstream, we use the imported *.impl.* definitions.
For convenience (not to move a lot of meson code around), define
the HAVE_GTK_WAYLAND and HAVE_GTK_X11 in config.h instead of
compiler flags.
This is code copied from xdg-desktop-portal-gnome repository today[0].
We need it to implement the FileChooser portal ourselves.
It's not included in the build yet, as it needs modifications first.
[0] b92a8cc5f6