build system (Perl bindings): Make Makefile.PL's PREFIX/INSTALL_BASE optional.

build system (Perl bindings): Make Makefile.PL's PREFIX/INSTALL_BASE optional.

Both options may cause problems:

 - PREFIX might clash with INSTALL_BASE being set in $PERL_MM_OPT.
 - INSTALL_BASE without any further options set will install into non-standard
   directories in many cases.

Rather, use PREFIX=${prefix} as default but overwrite that if
--with-perl-bindings=... has been used.

Thanks to faxm0dem for reporting the INSTALL_BASE problems on IRC and
bvarner and nirik for reporting this in GH #177 and #196.

This is a follow-up / fix for 97e8f346a88eccf4d1608c21d5cc6ee9620c1c41.

Fixes Github issue #290.

Signed-off-by: Florian Forster <octo@collectd.org>