Use Zig tool chain to cross compile C sshnpd for a variety of standard targets

[cpswan]

Is your feature request related to a problem? Please describe.

Present builds are slow for arm targets, and we don't offer musl based builds.

Describe the solution you'd like

If we make use of Zig cc to cross compile a CMake project it should be possible to speed up builds and offer a wider variety of targets.

Describe alternatives you've considered

There are probably ways to do this with the gcc cross compilers (and likely clang also).

[cpswan]

Putting P3 for now, as we don't have many folk using the binaries, and we're not yet doing frequent releases.

[cpswan]

Zig appears to work (for trivial cases), but doesn't get us a working sshnpd binary.

From the cpswan-zig-cc branch I've been able to create binaries that output help on their respective platforms, so the basics are there.

But when attempting a connection to the daemon it's unable to invoke srv with the command line being echoed to STDOUT e.g.:

[INFO] sshnpd | Starting main loop
[INFO] 2024-08-02 15:35:41.944013 | E2E TESTS | Monitor .*monitor started
[INFO] 2024-08-02 15:35:41.944013 | refresh_device_entry | Saving username entries for this device
[INFO] 2024-08-02 15:35:42.390470 | refresh_device_entry | Refreshing device info entries for this device
 srv -h 85.239.63.180 -p 43973 --local-port 22 --local-host localhost --rv-auth --rv-e2ee
[WARN] 2024-08-02 15:35:55.392265 | child_exit_handler | Received signal: -1501258096

Reading through zig cc: a Powerful Drop-In Replacement for GCC/Clang there's a section on compiler-rt which concludes:

Zig's compiler-rt is not yet complete. However, completing it is a prerequisite for releasing Zig version 1.0.0.

I suspect that how we invoke srv is trying to use some of the missing functionality.

For now, it would be prudent of us to wait until Zig hits v1.0.0 before returning to this approach.