[Makefile] avoid running make-os-ext unnecessarily
[Makefile] avoid running make-os-ext unnecessarily

mbk/ren-c,codebybrett/ren-c,rgchris/ren-c,codebybrett/ren-c,hostilefork/rebol,hostilefork/rebol,hostilefork/rebol,draegtun/ren-c,codebybrett/ren-c,mbk/ren-c,kealist/ren-c,codebybrett/ren-c,rgchris/ren-c,draegtun/ren-c,kealist/ren-c,rgchris/ren-c,hostilefork/rebol,giuliolunati/ren-c,draegtun/ren-c,giuliolunati/ren-c,giuliolunati/ren-c,rgchris/ren-c,kealist/ren-c,kealist/ren-c,draegtun/ren-c,hostilefork/rebol,draegtun/ren-c,mbk/ren-c,giuliolunati/ren-c,hostilefork/rebol,kealist/ren-c,giuliolunati/ren-c,rgchris/ren-c,kealist/ren-c,codebybrett/ren-c,mbk/ren-c,draegtun/ren-c,codebybrett/ren-c