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.

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.

compiler bugs in the Xcode toolchain.

crashing from an assertion failure.

flags that autoconf hasn’t checked first.

disabled until the engine code itself is ported to SDL2.

to external so as to avoid some autocompletion collisions with "configure".

that the OpenGL framework on OS X provides GL version 2 at least.