setup.py: simplify sys.platform != 'win32' detection and error reporting
setup.py: simplify sys.platform != 'win32' detection and error reporting

Signed-off-by: Dieter Verfaillie <ed2c9e2fab82acaa7daaf30cf5cc5ab110dfe77c@optionexplicit.be>

davidmalcolm/pygobject,MathieuDuponchelle/pygobject,nzjrs/pygobject,MathieuDuponchelle/pygobject,davibe/pygobject,jdahlin/pygobject,GNOME/pygobject,jdahlin/pygobject,davibe/pygobject,Distrotech/pygobject,choeger/pygobject-cmake,davidmalcolm/pygobject,sfeltman/pygobject,GNOME/pygobject,choeger/pygobject-cmake,GNOME/pygobject,thiblahute/pygobject,Distrotech/pygobject,sfeltman/pygobject,davibe/pygobject,alexef/pygobject,Distrotech/pygobject,pexip/pygobject,nzjrs/pygobject,pexip/pygobject,MathieuDuponchelle/pygobject,davibe/pygobject,sfeltman/pygobject,Distrotech/pygobject,davidmalcolm/pygobject,thiblahute/pygobject,thiblahute/pygobject,jdahlin/pygobject,alexef/pygobject,pexip/pygobject,alexef/pygobject,choeger/pygobject-cmake,nzjrs/pygobject