Add Nix flake

[svenssonaxel]

After much failure trying to follow BUILDING.txt instructions, I only managed to build using Nix. Contributing a flake.nix in case others have similar trouble.

• Supports one-line build for
  • linux x86_64
  • linux aarch64 (not tested)
  • darwin x86_64 (not tested)
  • darwin aarch_64 (not tested)
  • java
  • windows 64-bit
  • windows 32-bit

Marking as draft due to a couple of questions:

• NLS support for Windows (separate commit) creates a vncviewer.exe with a hard-coded path pointing to gettext language support files. Not sure why.
• Not sure if we want to support release builds using Nix.
• Windows builds are almost entirely static, using no *.dll files except for wm_hooks.dll. Do we want the other builds to also be static?

[CendioOssman]

I'm not too keen on another build script to maintain. So first I'd like to explore where you got stuck, and see if we can ease that hurdle?

[svenssonaxel]

Sure. I've already provided a Dockerfile explaining where I get stuck. Attached again here:

Dockerfile-debian.txt

Run it as docker build -f Dockerfile-debian.txt .. If you can edit this file so that building works, perhaps even simplify it, I think you'll have produced better documentation already.

I suppose my point is that this PR is not "another build script"; it's not an alternative to CMakeLists.txt. It is more like an executable description of dependencies, something that currently does not exist at all. The Dockerfile fulfills (or would, if fixed) the same function, in a less pure but perhaps more accessible way.