+ Rename some variables and modify a few definitions so that we match
  the same variables used in rubygem.mk.

+ Use a GENERATE_PLIST statement instead of manually generating a
  PLIST_DYNAMIC file.  This code is copied from RUBYGEM_GENERATE_PLIST
  in rubygem.mk.

+ Use INSTALL_ENV, which is meant to contain the extra environment
  bits for use during installation.

(jlam)