q3map2: workaround the Xonotic catharsis map build error (ugly workaround)

This workarounds the build failure of the Xonotic catharis map: #186

This is a very ugly workaround. I don't know what the code does, so it may silently produce unwanted side effects, or not, I don't know.

We should better fix the root cause of the problem, but I don't know what it is.

I seen no obvious wrongness after building catharsis with it.

It is assumed that the workaround only applies to maps affected by the bug, so the workaround should not break other maps in our back.

When the workaround is used, a fat and loud warning is printed remembering we should fix it for real. So if this workaround happens to be used with other maps, we may notice it.