Makefile.inc1: Require DISTDIR be set for make distributeworld

distributeworld is the target used to stage world for building dist sets
for release targets, and is not really intended for end-user use. If
DISTDIR is not set we would attempt to write to the root directory.
Error out in this case, serving as an assertion that an internal target
is not being used incorrectly.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48030