Update CMakeLists.txt to enable GLM Experimental

[christianhaitian]

Build fails on Debian Buster without these entries for either Release or Debug builds.

[tobbi]

In file included from /home/runner/work/supertux/supertux/src/audio/sound_manager.hpp:28:0,
                 from /home/runner/work/supertux/supertux/src/audio/openal_sound_source.cpp:19:
/home/runner/work/supertux/supertux/src/math/vector.hpp:20:0: error: "GLM_ENABLE_EXPERIMENTAL" redefined [-Werror]
 #define GLM_ENABLE_EXPERIMENTAL

Probably needs an #ifndef. Can you fix that as well?

[Grumbel]

Probably best to remove the #define GLM_ENABLE_EXPERIMENTAL from all the source and switch to just compiler flags.

-D GLM_ENABLE_EXPERIMENTAL shouldn't have a space.

[christianhaitian]

It compiled fine on my end with that inclusion as shown in the commit but I don't know what the implications are for non debian/ubuntu systems. If there's a more efficient solution for this that allows building on debian/ubuntu, I am in full support.

[Grumbel]

Fixed with b320c27