m4: update ax_pthread to latest
Latest version (serial 24) only updates the URL
diff --git a/m4/ax_pthread.m4 b/m4/ax_pthread.m4
index 4c4051e..5fbf9fe 100644
--- a/m4/ax_pthread.m4
+++ b/m4/ax_pthread.m4
@@ -1,5 +1,5 @@
# ===========================================================================
-# http://www.gnu.org/software/autoconf-archive/ax_pthread.html
+# https://www.gnu.org/software/autoconf-archive/ax_pthread.html
# ===========================================================================
#
# SYNOPSIS
@@ -67,7 +67,7 @@
# Public License for more details.
#
# You should have received a copy of the GNU General Public License along
-# with this program. If not, see <http://www.gnu.org/licenses/>.
+# with this program. If not, see <https://www.gnu.org/licenses/>.
#
# As a special exception, the respective Autoconf Macro's copyright owner
# gives unlimited permission to copy, distribute and modify the configure
@@ -82,7 +82,7 @@
# modified version of the Autoconf Macro, you may extend this special
# exception to the GPL to apply to your modified version as well.
-#serial 23
+#serial 24
AU_ALIAS([ACX_PTHREAD], [AX_PTHREAD])
AC_DEFUN([AX_PTHREAD], [
