flatpak: Build with meson

Meson is a fast-moving project, and while we don't require the very
latest version, the requirement is still recent enough to not
assume its general availability. However when building the flatpak,
we can easily ensure a recent enough version by bundling, so switch
the manifest to meson builds.

https://bugzilla.gnome.org/show_bug.cgi?id=779599