Fix pthread detection on Solaris 11.4 (#1479)

AX_PTHREAD requires _REENTRANT to confirm that pthread support is enabled.
Solaris 11.4 headers no longer use the macro, and GCC 16 therefore no
longer defines it for -pthread.

Detect XPG7 support in the target headers and require _REENTRANT only on
older Solaris releases. The existing pthread compile and link test remains
the final capability check.

This follows the canonical Autoconf Archive change:
https://github.com/autoconf-archive/autoconf-archive/pull/341

Regenerate configure with Autoconf 2.71.

Tested on Solaris 11.4 with GCC 15.2 and GCC 16.1. The Autoconf Archive
change was also tested on Solaris 11.3.

Co-authored-by: Rainer Orth <ro@CeBiTec.Uni-Bielefeld.DE>

ax_pthread.m4

@@ -87,7 +87,7 @@
 #   modified version of the Autoconf Macro, you may extend this special
 #   exception to the GPL to apply to your modified version as well.
 
-#serial 31
+#serial 32
 
 AU_ALIAS([ACX_PTHREAD], [AX_PTHREAD])
 AC_DEFUN([AX_PTHREAD], [
@@ -249,7 +249,22 @@ AS_IF([test "x$ax_pthread_clang" = "xyes"],
 # correctly enabled
 
 case $host_os in
-        darwin* | hpux* | linux* | osf* | solaris*)
+        solaris*)
+        # Solaris 11.4 introduced XPG7 support and did away with the need for
+        # _REENTRANT.
+
+        AC_EGREP_CPP([AX_PTHREAD_SOLARIS__REENTRANT],
+            [
+#            undef _XOPEN_SOURCE
+#            include <sys/feature_tests.h>
+#            if _XOPEN_VERSION < 700
+             AX_PTHREAD_SOLARIS__REENTRANT
+#            endif
+            ],
+            [ax_pthread_check_macro="_REENTRANT"],
+            [ax_pthread_check_macro="--"])
+        ;;
+        darwin* | hpux* | linux* | osf*)
         ax_pthread_check_macro="_REENTRANT"
         ;;
 

configure

@@ -18415,7 +18415,31 @@ fi
 # correctly enabled
 
 case $host_os in
-        darwin* | hpux* | linux* | osf* | solaris*)
+        solaris*)
+        # Solaris 11.4 introduced XPG7 support and did away with the need for
+        # _REENTRANT.
+
+        cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+#            undef _XOPEN_SOURCE
+#            include <sys/feature_tests.h>
+#            if _XOPEN_VERSION < 700
+             AX_PTHREAD_SOLARIS__REENTRANT
+#            endif
+
+_ACEOF
+if (eval "$ac_cpp conftest.$ac_ext") 2>&5 |
+  $EGREP "AX_PTHREAD_SOLARIS__REENTRANT" >/dev/null 2>&1
+then :
+  ax_pthread_check_macro="_REENTRANT"
+else $as_nop
+  ax_pthread_check_macro="--"
+fi
+rm -rf conftest*
+
+        ;;
+        darwin* | hpux* | linux* | osf*)
         ax_pthread_check_macro="_REENTRANT"
         ;;