The genstubs script has some code to prevent duplication of entries for macosx and unix items. Unfortunately it doesn't seem to work very well. If you create a unix item in tclInt.decls after slot 20 (listed for macosx), you lose the macosx items completely. This should not happen.
This needs to be corrected prior to 8.5.0 otherwise we will lost binary compat with any attempt to add more stubs in the currently broken generation script.