python2: Fix on non-Linux

The location of this 'regen' script (whatever is does) depends on the
platform. AFAICT it always exists due to this:

$(srcdir)/Lib/$(PLATDIR):
        mkdir $(srcdir)/Lib/$(PLATDIR)
        cp $(srcdir)/Lib/plat-generic/regen $(srcdir)/Lib/$(PLATDIR)/regen