switch to meson

We're currently maintaining two separate build-systems, one based on autotools, and one based on cmake. These are quite similar, but some details are missing from each of them, most notably:

• The autotools build-system can't build the src/wgl sources
• The cmake build-system doesn't install things properly

There's a bunch of other smaller differences, but they're not that important.

In theory we could have massaged the cmake build system to work well enough for all use-cases, but there's a big elephan in the room here:

Mesa's main repository has moved to meson. So if we keep doing cmake, the Mesa development community needs to learn two different build systems that fills the same needs. That's not really ideal.

So instead, let's add a Meson build system, and later on remove the other ones.

I think this is all; this should be a union of the feature-sets of the autotools and cmake build systems.

This also marks both the autotools and the cmake build as deprecated, requiring a configure-time option to opt-in. This is similar to what we did when transitioning Mesa.

Still needs some work on the macOS front, as xdemos doesn't seem to link without additional work...