Fall back on POSIX mkstemp() for systems lacking mkostemp()

We add an autotools check to use mkstemp() and fcntl() when mkostemp()
is missing. This is not strictly equivalent because we have no way of
setting the file access mode O_WRONLY flag, but that should produce no
functional difference.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Anthony G. Basile <blueness@gentoo.org>