WIP: Add meson.build files and GitLab CI

I wanted to switch eos-installer from Autotools to Meson, but realised that I'd also have to write Meson glue for libglnx. Here is that glue. I have not actually tested this as a Meson subproject so I am marking this as WIP. It builds and passes its tests on my Endless system. In particular:

Checking if "renameat2() is declared" compiles: YES
Checking if "memfd_create() is declared" compiles: YES
Checking if "copy_file_range() is declared" compiles: YES

Given these meson.build files, it's easy enough to add GitLab CI.