diff --git a/Modules/Platform/Darwin.cmake b/Modules/Platform/Darwin.cmake
index 4ec336478..035e37c1b 100644
--- a/Modules/Platform/Darwin.cmake
+++ b/Modules/Platform/Darwin.cmake
@@ -48,6 +48,12 @@ IF(NOT XCODE)
   SET(CMAKE_SHARED_LIBRARY_SONAME_Fortran_FLAG "-install_name")
 ENDIF(NOT XCODE)
 
+# Xcode does not support -isystem yet.
+IF(XCODE)
+  SET(CMAKE_INCLUDE_SYSTEM_FLAG_C)
+  SET(CMAKE_INCLUDE_SYSTEM_FLAG_CXX)
+ENDIF(XCODE)
+
 SET(CMAKE_MacOSX_Content_COMPILE_OBJECT "\"${CMAKE_COMMAND}\" -E copy_if_different <SOURCE> <OBJECT>")
 
 SET(CMAKE_C_CREATE_SHARED_LIBRARY_FORBIDDEN_FLAGS -w)