Set ZERO_AR_DATE=1 when running libtool.

test/mac/libtool-zero/mylib.c

Index: test/mac/libtool-zero/mylib.c
diff --git a/test/win/idl-excluded/program.cc b/test/mac/libtool-zero/mylib.c
similarity index 81%
copy from test/win/idl-excluded/program.cc
copy to test/mac/libtool-zero/mylib.c
index 9dc3c94f3431be00adb755f6515b995d734e089b..b26d61bd6ba91717a068fb4d6d4056f401473c68 100644
--- a/test/win/idl-excluded/program.cc
+++ b/test/mac/libtool-zero/mylib.c
@@ -2,6 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-int main() {
-  return 0;
+int my_foo(int x) {
+  return x + 1;
 }