Be more tolerant of certain autoconf-defined variables not being
available.

This can happen if needing to regenerate the autoconf goop on an
older system (such as CentOS 6), where PACKAGE_URL doesn't get
defined.
(cherry picked from commit 23c1008132757ee94f623e43511c9bbf94366dc8)

Signed-off-by: Bruce A. Mah <bmah@es.net>