We had some AC_CHECK_HEADER tests that were really wastes of cycles,
because the code proceeded to #include those headers unconditionally
anyway, in all or a large majority of cases.  The lack of complaints
shows that those headers are available on every platform of interest,
so we might as well let configure run a bit faster by not probing
those headers at all.

I suspect that some of the tests I left alone are equally useless, but
since all the existing #includes of the remaining headers are properly
guarded, I didn't touch them.