Conditional compilation of mongo_db_plugin.

plugins/CMakeLists.txt

@@ -9,6 +9,13 @@ add_subdirectory(account_history_plugin)
 add_subdirectory(account_history_api_plugin)
 add_subdirectory(wallet_plugin)
 add_subdirectory(wallet_api_plugin)
-add_subdirectory(mongo_db_plugin)
 add_subdirectory(txn_test_gen_plugin)
 add_subdirectory(faucet_testnet_plugin)
+
+find_package(libmongoc-1.0 1.8)
+
+if (libmongoc-1.0_FOUND)
+  add_subdirectory(mongo_db_plugin)
+else()
+  message("Could NOT find MongoDB. mongo_db_plugin will be omitted.")
+endif()