Try to find atomics only after setting up compiler flags, because the test will never work without C++11.

CMakeLists.txt

@@ -129,14 +129,14 @@ if (THREADS_HAVE_PTHREAD_ARG)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
 endif()
 
+include(TdSetUpCompiler)
+td_set_up_compiler()
+
 find_package(Atomics REQUIRED)
 if (ATOMICS_LIBRARY_FLAGS)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${ATOMICS_LIBRARY_FLAGS}")
 endif()
 
-include(TdSetUpCompiler)
-td_set_up_compiler()
-
 if (MSVC)
   option(TD_ENABLE_MULTI_PROCESSOR_COMPILATION "Use \"ON\" to enable multi-processor compilation.")