Teach it to scrape -I and -D switches from CPPFLAGS in Makefile.global.
This is useful for testing on, eg, FreeBSD, where you won't get far
without "-I/usr/local/include".

Also, expand the set of blacklisted-for-unportability atomics headers,
based on noting that arch-x86.h fails to compile on an ARM box.  The
other ones I'd omitted seem to compile all right on architectures they
don't belong to, but that's surely too shaky to rely on.  Let's do
like we did for the src/include/port/ headers, and ignore all except
the variant that's pulled in by the arch-independent header.