configure: fix detection of builtin libs with -Werror.

We didn't include stdlib, so these tests "failed" due to implicit
declaration of exit().

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

configure

@@ -169,7 +169,7 @@ int main(void)
 {
 	printf("%p\n", crypto_aead_chacha20poly1305_ietf_encrypt);
 	printf("%d\n", crypto_aead_chacha20poly1305_ietf_NPUBBYTES);
-	exit(0);
+	return 0;
 }
 /*END*/
 var=HAVE_SYSTEM_LIBBASE58
@@ -183,7 +183,7 @@ code=
 int main(void)
 {
 	printf("%p\n", b58check);
-	exit(0);
+	return 0;
 }
 /*END*/
 EOF