Autoconf 2.59d (released in 2006) [1] started promoting several macros
as not relevant for newer systems anymore, including the `AC_HEADER_DIRENT`.
This macro checks which header defines the `DIR` type. If `<dirent.h>`
is available it defines the `HAVE_DIRENT_H` symbol. Since the `<dirent.h>`
header is already checked in the `configure.ac`, this check is not needed
anymore. This macro also additionally checks for SCO Xenix (discontinued,
latest release 1989) dir and x libraries. [2]
Commit 6ed790685f introduced also
`<sys/dir.h>`. This header exists from times of UNIX System V and
provided definition of DIR type on these systems such as 4.3BSD.
Today `<sys/dir.h>` is kept for backwards compatibility and includes
the `<dirent.h>` on current systems. With `dirent.h>` present this
include is no longer required.
Refs:
[1] http://git.savannah.gnu.org/cgit/autoconf.git/tree/NEWS
[2] https://www.gnu.org/software/autoconf/manual/autoconf-2.69/autoconf.html
Remove unused dirent.h includes
