Add devshell for nix under macOs

[vkravets]

This PR added devshel under nix macOs and can be adopded in future for the linux with nix.

nix develop -c $SHELL

After this command enter to shell where all dependecies and configuration to build already defined.

README-OSX.md has beeen updated also.

This PR also added USE_SDL2 options which turn on by default on apple. Under brew (which is default build on macOs and easiest ) leagcy SDL is not avalaible and only sdl12-compat is available. Also there is deprecation for sdl-mixer and sdl-gfx but it's only disabled and can be enabled via brew edit, so I don't touch enything about it in this PR.

[vkravets]

As aditional for the https://github.com/MeridianOXC/OpenXcom/pull/123

[luqasz]

@vkravets I'd like to build on Mac OS via brew however sdl packages are not available anymore. README still state that it needs sdl but sdl2 and sdl3 are only available.