New extensions in glext.h follow the pattern:

    #ifndef GL_ARB_texture_rectangle
    #define GL_ARB_texture_rectangle 1
    #define GL_TEXTURE_RECTANGLE_ARB          0x84F5
    #define GL_TEXTURE_BINDING_RECTANGLE_ARB  0x84F6
    #define GL_PROXY_TEXTURE_RECTANGLE_ARB    0x84F7
    #define GL_MAX_RECTANGLE_TEXTURE_SIZE_ARB 0x84F8
    #endif /* GL_ARB_texture_rectangle */

Versions.h tries to declare:

    const char* const GL_ARB_texture_rectangle = "GL_ARB_texture_rectangle";

Which means, if you've included glext.h before Versions.h, that the
compiler will see "const char* const 1 = ...", and rightly refuse to
continue.

The ham-fisted approach taken here is to rename the variables in
Versions.h with a leading underscore.  This does sort of undermine the
comment about "better to have the compiler do spelling checks", but.
Signed-off-by: Adam Jackson <ajax@redhat.com>