libxfce4windowing check was a hard error even when Wayland support was
only auto-detected. Any X11 without libxfce4windowing would fail to
configure entirely.

This changes it to only error out if --enable-wayland=yes was explicit
and otherwise it just falls back to X11.

Also bumped the version requirement to 4.19.4, since that's what
XFW_CLIENT_TYPE_PAGER actually needs.

Fixes #1572

* Add Meson support

* Fix libegg submodule configure

* generate schemas before install