Add gmock into our dependencies.

third_party/boost/boost/preprocessor/facilities/empty.hpp

Index: third_party/boost/boost/preprocessor/facilities/empty.hpp
diff --git a/third_party/boost/boost/preprocessor/facilities/empty.hpp b/third_party/boost/boost/preprocessor/facilities/empty.hpp
new file mode 100644
index 0000000000000000000000000000000000000000..46db19026a9f4c5948cb6ed93afca94bd1977a9b
--- /dev/null
+++ b/third_party/boost/boost/preprocessor/facilities/empty.hpp
@@ -0,0 +1,21 @@
+# /* Copyright (C) 2001
+#  * Housemarque Oy
+#  * http://www.housemarque.com
+#  *
+#  * 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)
+#  */
+#
+# /* Revised by Paul Mensonides (2002) */
+#
+# /* See http://www.boost.org for most recent version. */
+#
+# ifndef BOOST_PREPROCESSOR_FACILITIES_EMPTY_HPP
+# define BOOST_PREPROCESSOR_FACILITIES_EMPTY_HPP
+#
+# /* BOOST_PP_EMPTY */
+#
+# define BOOST_PP_EMPTY()
+#
+# endif