consider extern fields featured only if they are actually used (fixed issue #1487)

git-svn-id: http://haxe.googlecode.com/svn/trunk@6209 f16182fa-f095-11de-8f43-4547254af6c6
[cpp] No dependence on external super files.  Do not generate code for external enums

git-svn-id: http://haxe.googlecode.com/svn/trunk@6210 f16182fa-f095-11de-8f43-4547254af6c6