Update autoconf archive macros

m4/ax_append_flag.m4

@@ -1,5 +1,5 @@
 # ===========================================================================
-#      http://www.gnu.org/software/autoconf-archive/ax_append_flag.html
+#      https://www.gnu.org/software/autoconf-archive/ax_append_flag.html
 # ===========================================================================
 #
 # SYNOPSIS
@@ -34,7 +34,7 @@
 #   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
@@ -49,21 +49,23 @@
 #   modified version of the Autoconf Macro, you may extend this special
 #   exception to the GPL to apply to your modified version as well.
 
-#serial 2
+#serial 7
 
 AC_DEFUN([AX_APPEND_FLAG],
-[AC_PREREQ(2.59)dnl for _AC_LANG_PREFIX
-AS_VAR_PUSHDEF([FLAGS], [m4_default($2,_AC_LANG_PREFIX[FLAGS])])dnl
-AS_VAR_SET_IF(FLAGS,
-  [case " AS_VAR_GET(FLAGS) " in
-    *" $1 "*)
-      AC_RUN_LOG([: FLAGS already contains $1])
-      ;;
-    *)
-      AC_RUN_LOG([: FLAGS="$FLAGS $1"])
-      AS_VAR_SET(FLAGS, ["AS_VAR_GET(FLAGS) $1"])
-      ;;
-   esac],
-  [AS_VAR_SET(FLAGS,["$1"])])
+[dnl
+AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_SET_IF
+AS_VAR_PUSHDEF([FLAGS], [m4_default($2,_AC_LANG_PREFIX[FLAGS])])
+AS_VAR_SET_IF(FLAGS,[
+  AS_CASE([" AS_VAR_GET(FLAGS) "],
+    [*" $1 "*], [AC_RUN_LOG([: FLAGS already contains $1])],
+    [
+     AS_VAR_APPEND(FLAGS,[" $1"])
+     AC_RUN_LOG([: FLAGS="$FLAGS"])
+    ])
+  ],
+  [
+  AS_VAR_SET(FLAGS,[$1])
+  AC_RUN_LOG([: FLAGS="$FLAGS"])
+  ])
 AS_VAR_POPDEF([FLAGS])dnl
 ])dnl AX_APPEND_FLAG