Enable Linux builds, ideally through a Github Action

[KeyWorksRW]

Description:

While I was never able to get any app that used wxWidgets to build on WSL2, I'm sure the problem is surmountable, and is probably mostly due to library conflicts. A bigger concern now is our requirement for a C++20 compiler. We need to check if the current version of gcc or clang that Github Actions uses can generate C++20 code. If so, then it's just a matter of fixing the library conflict, and setting up a correct setup.h file for Linux builds.