Cocoa: Fix compilation error on OS X 10.8

The occlusion state parts of NSWindow are OS X 10.9+ only but we still
(technically) support building for only 10.8.

Fixes #2161

README.md

@@ -273,6 +273,8 @@ information on what to include when reporting a bug.
    application (#2110)
  - [Cocoa] Bugfix: The Vulkan loader was not loaded from the `Frameworks` bundle
    subdirectory (#2113,#2120)
+ - [Cocoa] Bugfix: Compilation failed on OS X 10.8 due to unconditional use of 10.9+
+   symbols (#2161)
  - [X11] Bugfix: The CMake files did not check for the XInput headers (#1480)
  - [X11] Bugfix: Key names were not updated when the keyboard layout changed
    (#1462,#1528)

src/cocoa_window.m

@@ -311,10 +311,15 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
 
 - (void)windowDidChangeOcclusionState:(NSNotification* )notification
 {
-    if ([window->ns.object occlusionState] & NSWindowOcclusionStateVisible)
-        window->ns.occluded = GLFW_FALSE;
-    else
-        window->ns.occluded = GLFW_TRUE;
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= 1090
+    if ([window->ns.object respondsToSelector:@selector(occlusionState)])
+    {
+        if ([window->ns.object occlusionState] & NSWindowOcclusionStateVisible)
+            window->ns.occluded = GLFW_FALSE;
+        else
+            window->ns.occluded = GLFW_TRUE;
+    }
+#endif
 }
 
 @end