Update Ruby and Freeglut versions.
Fix typos.
Use function parameter declaration according to ANSI C.
Replace deprecated rb_thread_blocking_region() by rb_thread_call_with_gvl()
Link to both freeglut libraries and opengl, so that there are no unmet dependencies at build time for the windows build.
Add x64-mingw cross build.

Use mini_portile for building freeglut to simplify build tasks.