changes to lang/ruby/gem.mk

[rodent%NetBSD.org@localhost]

Can we add:

        a) s|${GEM_HOME}/cache/${GEM_NAME}.gem|${GEM_CACHEFILE}
        b) s|${GEM_HOME}/specifications/${GEM_NAME}.gemspec|${GEM_SPECFILE}
        c) PKGNAME?=${RUBY_PKGPREFIX}-${DISTNAME}

to lang/ruby/gem.mk needs, so that print-PLIST is smarter/shorter. url2pkg
will then not set PKGNAME for URLs ending in .gem.