We now just look for SDL2/SDL.h (the regular location) and SDL.h (when
the user provides their own location for this header).

The only reason we had to keep this was for emscripten builds. We can
now use SDL v2 using “emconfigure ./configure CFLAGS="-s USE_SDL=2” and
a few additional flags.

SDL_Mixer v2 is not available in emscripten packages but either it will
be added at some time, or I will get rid of it altogether. An alternate
solution could be this code snippet I found:

    cd SDL2_mixer
    emconfigure ./configure --prefix=$(pwd)/dist --enable-music-ogg --disable-music-ogg-shared CFLAGS="-s USE_VORBIS=1"
    emmake make install

This flag disables GL, GLES, but also Glew and EGL libraries.

Linux-only for now. This will be necessary for projects that do not
actually use the rendering engine, such as LolRemez.

All these changes will allow us to more easily add Lol Engine as a
submodule of another Git project.

If SDL_CFLAGS is defined to a non-empty value, even spaces, a later
test will wrongly succeed and think SDL_CFLAGS is a user override.

disabled until the engine code itself is ported to SDL2.