Define _NETBSD_SOURCE and _OPENBSD_SOURCE in AC_USE_SYSTEM_EXTENSIONS (#110392)

Define _NETBSD_SOURCE and _OPENBSD_SOURCE in AC_USE_SYSTEM_EXTENSIONS (#110392)

These expose additional extensions specific to those operating
systems, similar to _DARWIN_C_SOURCE, _GNU_SOURCE, etc.

(DragonflyBSD and FreeBSD currently do not have any equivalent
macros.)

Fixes bug #110392.  See also
https://git.savannah.gnu.org/cgit/gnulib.git/tree/m4/extensions.m4
https://git.eyrie.org/?p=devel/rra-c-util.git;a=commitdiff;h=f8a922cf31804dcc25ac176dcc22fdcdffcb5fdf

* lib/autoconf/specific.m4 (AC_USE_SYSTEM_EXTENSIONS): Also define
  _NETBSD_SOURCE and _OPENBSD_SOURCE.  Add comment explaining that
  there are (currently) no equivalent macros on DragonflyBSD and
  FreeBSD.  Put macro list in alphabetical order.