Find correct place for GL_GLEXT_PROTOTYPES header definition

GL_GLEXT_PROTOTYPES was added quick and dirty into the SDL header for now (where it probably doesn't belong in the first place). Evaluate what the correct header for this define is (probably glext.h) and move the define.

[juj]

Unassigning old issue. The next step on the old SDL 1.x headers will be to remove those from the codebase, if something.

[stale]

This issue has been automatically marked as stale because there has been no activity in the past year. It will be closed automatically if no further activity occurs in the next 30 days. Feel free to re-open at any time if this issue is still relevant.