Makefile.inc1: Require DISTDIR be set for `make distributeworld`

[emaste]

Summary

distributeworld is the target used to stage world for building dist sets
for release targets.  Error out if attempting to distributeworld to /.

[brooks]

This seems like a good change. I have some vague memories about complaints when I changed things in this area, but I think that might have been the largely unrelated "distribution" target.

[imp]

Yes. Good change. I've shot my foot off this way