Martin's change 43604 broke the Mac builds apparently due to an autoconf

Martin's change 43604 broke the Mac builds apparently due to an autoconf
bug.  I don't understand this at all, but Darwin/[78].* gets converted
to Darwin/78.* which is not correct.  Maybe I'm just clueless or overworked.
I can't see why in the original checkin this should have changed.

This hack gets the Mac build working again.  If someone figures out the
real problem, please revert this and fix for real.

Anthony is telling me that AC_PROG_CXX_WORKS which we use is broken.
I have no idea if that's related.

This change breaks up the case and fixes a typo.