DF unconditionally uses OpenGL and GLU

[rofl0r]

which is not available everywhere.
rather it should use plain SDL as a fallback, which gets used when dlopen() of GL stuff fails
(linux)

[rofl0r]

i've addressed this in my nogl branch https://github.com/rofl0r/df-libgraphics/commits/nogl

however i cannot test if it works as intended due to a segfault in dwarf fortress binary which i'm unable to debug.
i suspect the segfault is caused by different libstdc++ versions.