Define _XOPEN_SOURCE in a single place and mention illumos

Move the Solaris definition into the same conditional as the general one,
so that all _XOPEN_SOURCE decisions are together.  The socket ancillary
data API is gated on _XOPEN_SOURCE only on illumos; Solaris declares it
anyway.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Misc/NEWS.d/next/Build/2026-07-20-20-30-00.gh-issue-154271.solxopen.rst

@@ -1,4 +1,4 @@
-Define ``_XOPEN_SOURCE=600`` on Solaris and illumos, so that the socket
-module is built with :meth:`~socket.socket.sendmsg`,
+Define ``_XOPEN_SOURCE=600`` on Solaris and illumos, so that on illumos the
+socket module is built with :meth:`~socket.socket.sendmsg`,
 :meth:`~socket.socket.recvmsg` and the ``CMSG_*`` helpers.  This also
 enables the ``forkserver`` :mod:`multiprocessing` start method.

configure

@@ -871,8 +871,8 @@ case $ac_sys_system/$ac_sys_release in
   # Marc Recht
   NetBSD/1.5 | NetBSD/1.5.* | NetBSD/1.6 | NetBSD/1.6.* | NetBSD/1.6@<:@A-S@:>@)
     define_xopen_source=no;;
-  # On Solaris, _XOPEN_SOURCE disables features unless __EXTENSIONS__ is
-  # also defined; a dedicated _XOPEN_SOURCE=600 is set for it below.
+  # On Solaris, _XOPEN_SOURCE=800 hides platform specific features.
+  # A lower level is defined below.
   SunOS/*)
     define_xopen_source=no;;
   # On UnixWare 7, u_long is never defined with _XOPEN_SOURCE,
@@ -938,12 +938,12 @@ then
   # with -std=c11.  Defining _ISOC23_SOURCE makes them visible again.
   AC_DEFINE([_ISOC23_SOURCE], [1],
             [Define to activate ISO C23 library declarations])
-fi
-
-# On Solaris the socket ancillary-data API (CMSG_*, sendmsg(), recvmsg())
-# needs _XOPEN_SOURCE >= 600; __EXTENSIONS__ keeps the other platform
-# features.  This enables the forkserver start method.  See gh-57208.
-if test "$ac_sys_system" = "SunOS"; then
+elif test "$ac_sys_system" = "SunOS"
+then
+  # On illumos the socket ancillary-data API (CMSG_*, sendmsg(), recvmsg())
+  # is declared only with _XOPEN_SOURCE >= 600; Solaris declares it anyway.
+  # __EXTENSIONS__ keeps the platform specific features which _XOPEN_SOURCE
+  # would otherwise hide.  See gh-57208.
   AC_DEFINE([_XOPEN_SOURCE], [600],
             [Define to the level of X/Open that your system supports])
 fi