Motivation for this change

Build of the package devilutionx fails.

The error message:

make[2]: *** No rule to make target '/nix/store/slyxpxxprgdq3x54nanqg7sd3wg1cks7-SDL2-2.0.12/lib/libSDL2main.a', needed by 'devilutionx'.  Stop.

From my limited understanding of the build process, this is due probably to this commit, which stopped moving /libSDL2main.a to $dev folder.

The same commit added a possibility of removing *.la files insteaf of *.a files. It can be done by setting dontDisableStatic flag, but curiously it is not possible to set this flag for SDL2. Or maybe I'm missing something...

This PR adds a new parameter to SDL2: withStatic ? false. It is used to set dontDisableStatic = withStatic;. By default, if not used, it won't change the current behaviour. Setting it to true for the build of devolutionx fixes the build on my machine.

dontDisableStatic = withStatic; exists in other packages, and inspired this solution.

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.

Pinging @cpages, the maintainer, and @nh2, who committed 57908c1