Add a buildroot option to gem install.

This option helps packagers, who want to repackage gems in deb/rpm.
It is also needed to make tools/rbinstall.rb work in ruby 2.2.
Without this patch and another small patch for rbinstall.rb, "make
install" ignores DESTDIR when installing the stdlib gems.
Merge pull request #965 from darix/add_build_root_option

Add a buildroot option to gem install.