add helpful error message if gtest isn't set up

c++/CMakeLists.txt

@@ -3,8 +3,13 @@ cmake_minimum_required(VERSION 2.8)
 
 set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wno-unused-parameter -std=c++11")
 
-add_subdirectory(gtest)
-include_directories(${CMAKE_CURRENT_SOURCE_DIR}/gtest/include)
+if(EXISTS "${dir}" AND IS_DIRECTORY "${dir}")
+  add_subdirectory(gtest)
+  include_directories(${CMAKE_CURRENT_SOURCE_DIR}/gtest/include)
+else()
+  message(FATAL_ERROR "Please use setup-gtest.sh to download gtest before running the cmake build")
+endif()
+
 include(CTest)
 
 add_subdirectory(src)
\ No newline at end of file