libwebsockets: improve cross-compilation support

[Mindavi]

Building works on aarch64-multiplatform:

• libwebsockets_3_1
• libwebsockets_3_2
• libwebsockets_4_0
• libwebsockets_4_1

This patch disables building the test applications for libmosquitto in case you're cross-compiling. I don't think they're that important and this is the easiest fix for enabling cross-compilation.

If anyone cares enough about the test applications and wants to use them in a cross scenario, they could properly patch this. I'd assume that a patch would be the same for all versions, since I've seen this error at least on 3.1 and 4.1.

Error log without this patch

rick@nixos-asus:~/hobby/nixos-mobile-nixpkgs$ nix build https://github.com/nixos/nixpkgs/archive/master.tar.gz#pkgsCross.aarch64-multiplatform.libwebsockets_4_0
error: --- Error ----------------------------------------------------------------------------------------------- nix
builder for '/nix/store/lc966irnncjnppdnj4j6icpsbmkyjqvx-libwebsockets-4.0.21-aarch64-unknown-linux-gnu.drv' failed with exit code 2; last 10 log lines:
  -- Installing: /nix/store/z1h2gkdiqyqpp9fdpabf369pvqmrh387-libwebsockets-4.0.21-aarch64-unknown-linux-gnu/bin/libwebsockets-test-server-extpoll
  -- Installing: /nix/store/z1h2gkdiqyqpp9fdpabf369pvqmrh387-libwebsockets-4.0.21-aarch64-unknown-linux-gnu/bin/libwebsockets-test-lejp
  -- Installing: /nix/store/z1h2gkdiqyqpp9fdpabf369pvqmrh387-libwebsockets-4.0.21-aarch64-unknown-linux-gnu/bin/libwebsockets-test-client
  CMake Error at cmake_install.cmake:174 (file):
    file INSTALL cannot find
    "/build/source/build/libwebsockets-test-server.key.pem": No such file or
    directory.
  
  
  make: *** [Makefile:147: install] Error 1

Motivation for this change

libwebsockets is a dependency of mosquitto, and I tried cross-compiling mosquitto. It was easy to fix this dependency, so here we are.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Note that I haven't actually tried running this (yet), but at least it compiles now, which is a step in the right direction IMO.

[SuperSandro2000]

pinging @fabaff because there is no maintainer. Maybe one of you wants to add themselves?