cpanminus: fix installation paths

perlcross seems to set the installation paths differently than perl's
Configure, so adapt the reference to these paths in cpanminus.

[Francois: install into /usr/lib/perl]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>