Add "lib" directory to PERL5LIB
As part of https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=28489, I'm planning to add a "lib" directory to the source tree, and we'll need this as part of koha-gitify, so that Apache sets PERL5LIB correctly.
Having non-existent directories in PERL5LIB is fine, so this change is backwards compatible.