Executables that use libbullet are now 1 MiB smaller (stripped!).

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

Sometimes libEGL.so can be available (provided as a symlink
by Debian diversion packages) while EGL/egl.h isn’t.

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 gives us a reasonably cross-platform way to to build options. Works
well with LolRemez, where we no longer depend on GUI libraries.

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

When built as a subproject, we need the master to decide what features
to activate or not. This could lead to reduced dependences.

This patch also allows to build without imlib2.

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

This will help us deal with shader name collision. Breaks the Windows
build for the moment, unfortunately.

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.