Don't call CPAN to install any "extra" modules for the install-notify target.

Previously the Makefile's install-notify target built extra.install just like
the other 5 install targets. However, the other 5 actually install perl,
whereas install-notify (a.k.a. no-install) runs installperl and installman
with the --notify option (formerly -n), to cause them to print what they would
do but take no action. Hence if -Dextras was used, make no-install would
actually install the extra modules, but nothing else. This is inconsistent.