Uses pkg-config to source locations of libSDL.
As commented in Makefile, *may* cause issues with builds on Mac, but
there are known Mac-isms to make this work.
Also needs the math library for round() as referenced by video.o
This causes builds to start working on Ubuntu 24.04 with the default
SDL2 libraries installed using apt.
Arch is known to need pkg-config to use SDL libs too, but I'm not
sitting at my arch machine at the moment, so no idea if this is complete
for that platform.
A meta-build system *may* be a good idea?