general: add support for Meson

Since it’s 2017 already, Nautilus should use a build system that doesn’t
take longer to set up the build than it takes to actually build. An
observed build time using Ninja of roughly one-fifth of what it took
Autotools is more than reason enough to add support for Meson. Along
with that, this commit adds a convenience script to generate a tarball
for releases, since we use libgd as a submodule and Meson does not
handle source distributions.

https://bugzilla.gnome.org/show_bug.cgi?id=778167