Fred:
...
> The compile fails because SDL.h can't be found.
> Package sd12 was not found in the pkg-config search path. It also
> mentions adding directory containing sd12.pc to the PKG-CONFIG_PATH
> environment variable.

To find which package a missing file belongs to, do:

# apt-get install apt-file

$ apt-file search SDL.h
$ apt-file search sdl2.h

///

  I get:
$ apt-file -x search /SDL.h$
libsdl1.2-dev: /usr/include/SDL/SDL.h     
libsdl2-dev: /usr/include/SDL2/SDL.h
$ apt-file search /sdl2.pc
libsdl2-dev: /usr/lib/x86_64-linux-gnu/pkgconfig/sdl2.pc

So, the above problem is solved by installing libsdl2-dev.

Regards,
/Karl Hammar