diff --git a/src/OpenGl/OpenGl_Context_1.mm b/src/OpenGl/OpenGl_Context_1.mm
index 0c5badad5d..f5b5cf723f 100644
--- a/src/OpenGl/OpenGl_Context_1.mm
+++ b/src/OpenGl/OpenGl_Context_1.mm
@@ -15,6 +15,8 @@
 
 #if defined(__APPLE__) && !defined(MACOSX_USE_GLX)
 
+#define GL_GLEXT_LEGACY // To prevent inclusion of system glext.h on Mac OS X 10.6.8
+
 #import <Cocoa/Cocoa.h>
 
 #include <OpenGl_GlCore11.hxx>
diff --git a/src/OpenGl/OpenGl_Window_1.mm b/src/OpenGl/OpenGl_Window_1.mm
index 59444040ef..2ac4f5d746 100644
--- a/src/OpenGl/OpenGl_Window_1.mm
+++ b/src/OpenGl/OpenGl_Window_1.mm
@@ -15,6 +15,8 @@
 
 #if defined(__APPLE__) && !defined(MACOSX_USE_GLX)
 
+#define GL_GLEXT_LEGACY // To prevent inclusion of system glext.h on Mac OS X 10.6.8
+
 #import <Cocoa/Cocoa.h>
 
 #include <InterfaceGraphic.hxx>