Index: llvm/cmake/modules/CheckCompilerVersion.cmake
===================================================================
--- llvm/cmake/modules/CheckCompilerVersion.cmake
+++ llvm/cmake/modules/CheckCompilerVersion.cmake
@@ -27,6 +27,7 @@
         set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
         set(OLD_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
         set(CMAKE_REQUIRED_FLAGS "-std=c++0x")
+        INCLUDE(CheckCXXSourceCompiles)
         check_cxx_source_compiles("
 #include <atomic>
 std::atomic<float> x(0.0f);