If we're going to test for switch validity by observing whether the

compiler emits any warnings, the test program had better be 100%
correct, not only 90% correct.  The recent addition of -Wold-style-definition
broke thread-safety detection on every platform that has that switch,
because the test program used an old-style definition.

config/acx_pthread.m4

@@ -134,8 +134,10 @@ for flag in $acx_pthread_flags; do
             # We find them by checking stderror.
             cat >conftest.$ac_ext <<_ACEOF
 int
-main ()
+main (int argc, char **argv)
 {
+  (void) argc;
+  (void) argv;
   return 0;
 }
 _ACEOF

configure

@@ -13606,8 +13606,10 @@ rm -f conftest.$ac_objext conftest$ac_exeext conftest.$ac_ext
             # We find them by checking stderror.
             cat >conftest.$ac_ext <<_ACEOF
 int
-main ()
+main (int argc, char **argv)
 {
+  (void) argc;
+  (void) argv;
   return 0;
 }
 _ACEOF