+ Modify gem-rake-build to find the generated .gem file in the event
  that the Rakefile creates it in a non-standard place.

+ Create a new variable GEM_CLEANBUILD which is a list of globs for
  files to remove from the gem installed in the buildroot in the
  _gem-install-cleanbuild target.  This allows for packages with a
  non-standard gem directory structure (one that has the .c files
  under ext/).

(jlam)