Added a pkg-config file and copied the relevant source file into the
nix store. The idea is that the user may now relatively easily include
the library’s source file in their project using common CMake features.

Motivation for this change

The easyloggingpp package used to be header-only, but long compile times motivated a change to move as much code as possible into a C++ source file that could be compiled separately. Suggested use of the library involves either:

• Link to a static library compiled from the C++ source file
• Include the C++ source file in your own project

The former is not preferred in nixpkgs (or several other distros), as @Mic92 rightfully pointed out in #30256

The latter has an additional advantage in that several pre-processor definitions may be used to control the logging library's behavior if one recompiles the source file.

To support including the source file in what I hope is a nixpkgs-friendly way, I create a pkg-config file for the project during installation so that including the source file in a project built by cmake involves something like the following,

pkg_check_modules(EASYLOGGINGPP REQUIRED easyloggingpp)
add_executable(main src/main.cpp ${EASYLOGGINGPP_PREFIX}/src/easylogging++.cc)

That example is included in a comment in the nix package definition.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.