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.