Only attempt to build extensions for newly-installed gems. This
prevents compilation attempts at gem activation time for gems that
already have extensions built.
Fix crash in the dependency resolver for dependencies that cannot be
resolved.
* test/rubygems: ditto.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43368 b2dd03c8-39d4-4d8f-98ff-823fe69b080e