Add Meson build system

We want the Meson build system in Weston for going forward. Once it's in, let's do a release with both autotools and Meson before removing autotools.

Meson should make the build system much more approachable and easier to enhance.

Build options handling should not follow autotools to the letter, but it should be made more clean and easier to understand (especially the options around building or installing clients). Since the syntax changes for meson, might rework the options as well. If there is an option to not install something, that could as well not build it either.