Change to a consistent system for checking OpenGL extensions including handling AIX and Win32 by replacing all the symbols with function pointers. This also checks that the extension is available and that all the symbols are loadable, either from dlsym or GLX_ARB_get_proc_address if available.  The Graphics_library_check_extension macros use a flag to ensure the extension is checked for once only. Added a check so that if we are compiling with EXTENSION_FUNCTION_HANDLES but we do not have the type definitions, things still compile but the extensions are disabled.
Change to a consistent system for checking OpenGL extensions including
handling AIX and Win32 by replacing all the symbols with function pointers.
This also checks that the extension is available and that all the symbols are loadable,
either from dlsym or GLX_ARB_get_proc_address if available.  The
Graphics_library_check_extension macros use a flag to ensure the extension is
checked for once only.
Added a check so that if we are compiling with EXTENSION_FUNCTION_HANDLES but
we do not have the type definitions, things still compile but the extensions
are disabled.


cmiss/cmgui,cmiss/cmgui