WIP: Add Meson build system

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), and now me.

The build aims to be equivalent to autotools with the following exceptions:

• There is no option to use wayland-scanner from outside. For cross-compiling, it will be built twice instead: once for build, another for host for installing.

Fixes: #80 (closed)