Add gmock into our dependencies.

third_party/boost/boost/preprocessor/empty.hpp

Index: third_party/boost/boost/preprocessor/empty.hpp
diff --git a/third_party/boost/boost/preprocessor/empty.hpp b/third_party/boost/boost/preprocessor/empty.hpp
new file mode 100644
index 0000000000000000000000000000000000000000..116ef744e042bcf864e73afa2f9f2bdf6d8f4a98
--- /dev/null
+++ b/third_party/boost/boost/preprocessor/empty.hpp
@@ -0,0 +1,17 @@
+# /* **************************************************************************
+#  *                                                                          *
+#  *     (C) Copyright Paul Mensonides 2002.
+#  *     Distributed under the Boost Software License, Version 1.0. (See
+#  *     accompanying file LICENSE_1_0.txt or copy at
+#  *     http://www.boost.org/LICENSE_1_0.txt)
+#  *                                                                          *
+#  ************************************************************************** */
+#
+# /* See http://www.boost.org for most recent version. */
+#
+# ifndef BOOST_PREPROCESSOR_EMPTY_HPP
+# define BOOST_PREPROCESSOR_EMPTY_HPP
+#
+# include <boost/preprocessor/facilities/empty.hpp>
+#
+# endif