[PATCH xserver 1/9] First round of meson fixes

[Adam Jackson]

This fixes the dmx build, and builds the rest of the Xorg loadables.

I kind of wish static library targets would accept an empty file list,
I think the 'if srcs_libc.length() > 0' trick in os/meson.build is a
bit ugly. Ah well, the fact that you can then pass it as a link_with to
another static library and have it work like you expect makes up for it.

- ajax