Add Meson build system
This is a continuation of #3, updated with a few fixes.
Feedback welcome!
Meson is easier to work with than autotools, and has better support for cross-compilation. It also seems to make integrating ASan possible, unlike autotools.
This is based on the original Meson port by @ebassi (https://patchwork.freedesktop.org/patch/216512/, https://patchwork.freedesktop.org/series/42121/), continued by @daniels (https://gitlab.freedesktop.org/daniels/wayland/commits/wip/2018-08/meson-ci-asan), then @pq (!3 (closed)) and now me.
The build aims to be equivalent to autotools.
Fixes: #80 (closed)