Report missing git submodules from CMake.

CMakeLists.txt

@@ -19,6 +19,10 @@ set(CMAKE_C_STANDARD 99)
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
+if(NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/googletest/.git")
+  message(FATAL_ERROR "submodules not initialized: run\n\tgit submodule update --init --recursive\nin base git checkout")
+endif()
+
 add_definitions(-DGTEST_HAS_TR1_TUPLE=0)
 set(gtest_force_shared_crt ON CACHE BOOL "")
 add_subdirectory(googletest)