Include googletest as a submodule (#3993)

Add googletest as a submodule in third_party/googletest.
diff --git a/configure.ac b/configure.ac
index 4e3dc61..6283e31 100644
--- a/configure.ac
+++ b/configure.ac
@@ -214,7 +214,7 @@
 #   too.
 export CFLAGS
 export CXXFLAGS
-AC_CONFIG_SUBDIRS([gmock])
+AC_CONFIG_SUBDIRS([third_party/googletest])
 
 AC_CONFIG_FILES([Makefile src/Makefile benchmarks/Makefile conformance/Makefile protobuf.pc protobuf-lite.pc])
 AC_OUTPUT