Update comment related to C99

Update comment related to C99

One could do more work here to eliminate the Windows difference
described in the comment, but that can be a separate project.  The
purpose of this change is to update comments that might confusingly
indicate that C99 is not required.

Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/170308e6-a7a3-4484-87b2-f960bb564afa%40eisentraut.org