Fix loading of extension functions on OS X.

For a non-GLX application, at least, we need to pull our extensions
using dlsym, not glXGetProcAddress().  The symptom was a segfault in
glXGetProcAddress().
Fix declaration of glBegin/End_unwrapped.

On non-win32, it was declaring a new function pointer variable on
every #include, and OS X's linker got angry.
Fix printout of missing versions/extensions.

Not sure how I misplaced the \n in the previous fix.