app.nim: Add selection of display server

[fleischie]

Add compilation parameters to select enabled display servers. This commit prepares the sokol header's wayland-support feature.

The options are:

• disable_x11, and
• disable_wayland.

And can be used via -d:disable_x11 and/or -d:disable_wayland.

I figured when wayland-support for sokol drops, both X11 and wayland will be compiled in, and can be disabled individually, instead of explicitly enabled. This is not the ideal solution for these bindings currently, as there will be new libraries necessary that aren't used, yet. I didn't want to have a disable_x11 and an enable_wayland flag, though.

Edit: Depending on the order in which they are merged, I will keep this and PR #15 updated, so that both merge cleanly onto the main branch.

[floooh]

Looks like the Wayland dev libraries need to be installed, you can try adding those to the PR branch here:

https://github.com/floooh/sokol-nim/blob/1cd47e85856552563b0a742600238089936e95b5/.github/workflows/main.yml#L17

...but I'm not sure if GH Actions picks up the workflow file from the branch (otherwise, just give me the list of packages and I can add them to the main branch)