build: Vendor our own copy of wflinfo

This was the last remaining third-party executable used for our diagnostic checks that needed its packaging to be forked to build a multiarch-qualified version. If we avoid needing that, then we can use dependencies from an ordinary Linux distribution, making development outside a Steam Runtime environment easier.

The next step for this would be to switch helpers/check-gl.c to use libwaffle to create windows and contexts so that it can support more platforms than just X11/GLX, and incorporate the parts of wflinfo's functionality that we need into check-gl, so we only need to run one check program rather than two.

---

Various compiler warning fixes forwarded to https://gitlab.freedesktop.org/mesa/waffle/-/merge_requests/100, https://gitlab.freedesktop.org/mesa/waffle/-/merge_requests/101, https://gitlab.freedesktop.org/mesa/waffle/-/merge_requests/102.