Re: SDL3 support

[Alfred M. Szmidt]

Looking forword to testing this! Nice!

   I also changed the Makefile. I think it is not nice to change
   Makefile to build with different backends, so I changed this to a
   variable. There is no default so it has to be specified with make
   or as an environment variable. For example: make USIM_BACKEND=sdl3
   (or sdl2, sdl1, x11). I also added things to Makefile for creating
   the dependency files (%.d). Let me know if you do not want Makefile
   to be like this.

I can give you the reason why it was written like that at least.
ifeq, etc only work with GNU make.  And the Makefile was purposefully
written so that it would work on OpenBSD too.