Meson for XTS

This is a port of the autotools build system to meson, I've tried to be minimally invasive to the source code as possible, although there's been a few cases I've needed to. This is quite fast due to meson being faster than autotools (and not checking for flags we can't use), and ninja being smarter than make and not recursing.

I've learned a lot about how not to write C code from this exercise, it was quite interesting. Due to the number of warnings that seem to be part of the intentional coding style I've added '-w' to the build so it will shut up, though I've left the standard Xorg warn flags commented out so that someone brave can fix this pile.

I think everything is working save the docs, which I haven't gotten to yet.

This needs this piglit MR for xts/piglit integration to work.