Adapt configure test to work with -Werror and -Wunused-but-set-variable

Index: configure
--- configure.orig
+++ configure
@@ -13793,7 +13793,7 @@ $as_echo_n "checking for the pthreads library -l$flag.
         cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 #include <pthread.h>
-                        static void routine(void *a) { a = 0; }
+                        static void routine(void *a) { (void)a; }
                         static void *start_routine(void *a) { return a; }
 int
 main ()
