Log Message:
Force detection of grep and egrep by the configure script.

Unfortunately, configure doesn't always manage to correctly pick up these
values from GREP and EGREP, so we pass them in as cached values. This way,
configure still doesn't always print them correctly, but they end up in
config.status, which is the end goal.

This solves the problem where on Solaris, configure detects the wrong grep
and worse still, fails to detect a suitable egrep at all.