better tests for feature test macro permutations?

[Roland McGrath <roland at hack dot frob dot com>]

Bug 13775 suggests that at some point merely:

	#define _GNU_SOURCE
	#include <stdio.h>

had a compilation error on some machines (powerpc, s390, and sparc, I think).

I'm not really clear on when this could have been the case.  The bug was
reported for 2.13, but I don't see any features.h changes since then that
would explain how __USE_XOPEN2K8 could have failed to be defined under
_GNU_SOURCE.  But I'll admit I didn't really try to puzzle through the nest
of conditions thoroughly.

I mention this because it made me wonder how this slipped through.
Obviously it only affected a few machines (__LDBL_COMPAT).  But I'm pretty
sure somebody ran make check on s390 and powerpc at numerous times in
between.  So it seems likely we need some tests that attempt including
various headers under various permutations of feature test macros to ensure
that they all compile.

Joseph just pointed out that _FORTIFY_SOURCE is another relevant one here,
so we should be sure to include that in the set we try to cover.


Thanks,
Roland