Clang-6 and GNUisms.

[Mark Linimon]

The problem is even worse on armv6/armv7/aarch64, and much worse on
powerpc64/sparc64, which still have gcc in base.

I have not been saving up the emails where ports committers have been
fixing various failure modes.  I hesitate to start making harmless-
seeming patches myself for fear of my non-existant C++ skills.

I would be glad to help someone write up some documentation.  This
affects hundreds of port builds right now -- I have not even caught
up yet on tracking them all.

mcl