original header is not available on the system.

As agreed with Sulekha. Conditionalizing the installation via CMake
might be even better, but that would take some more thought, especially
if we still want to test threads_checked.h on systems that don't have
threads.h.

Note that for the wrapper headers (`foo.h` as opposed to
`foo_checked.h`), we reverse the order of the conditionals because we
always want to use the `#error` message from `foo.h`, not the one from
`foo_checked.h`, which wouldn't make sense if the user originally
included `foo.h`.

The `#error` message is subject to change in Microsoft's code review.

This applies the change from
microsoft#309 to support the common use
case for sprintf-like functions with null-terminated arrays.

This is analogous to microsoft#298 for
`free` and presumably is safe for the same reason.

Co-authored-by: John Kastner <john@correctcomputation.com>
Co-authored-by: mwhicks1 <mwh@correctcomputation.com>

Originally by Mike with no stated reason. We're discussing whether we
want to keep it.