extmk.rb: no EXTOBJS to main if enable-shared

* ext/extmk.rb: as all extension objects including initializations
  of ext and enc shouild be linked to libruby if enable-shared,
  EXTOBJS should not be linked to main programs.
  [ruby-core:66675] [Bug #10566]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48711 b2dd03c8-39d4-4d8f-98ff-823fe69b080e