The AMD_seamless_cubemap_per_texture extension refers to the token TEXTURE_CUBE_MAP_SEAMLESS_ARB. The enumext.spec file also refers to this token.
However, TEXTURE_CUBE_MAP_SEAMLESS_ARB does not exist. ARB_seamless_cube_map is a *core* extension, so its enums and function names do not have the ARB suffix. The proper enum is TEXTURE_CUBE_MAP_SEAMLESS.
This leaves the line in the enumext.spec file looking like this:
use ARB_seamless_cube_map TEXTURE_CUBE_MAP_SEAMLESS_ARB
This is a bad reference, since the enum isn't spelled that way. Both the AMD extension specification and the enumext.spec file need to be updated.
Reassigning to Graham to update the extension spec. I'll fix the headers