Cocoa: Patched use of macOS 10.9 window occlusion API for macOS 10.8 compatibility

This fixes building against macOS SDKs older than 10.9, noted in #2161.

CONTRIBUTORS.md

@@ -124,6 +124,7 @@ video tutorials.
  - Marco Lizza
  - Eyal Lotem
  - Aaron Loucks
+ - Ned Loynd
  - Luflosi
  - lukect
  - Tristam MacDonald

README.md

@@ -269,6 +269,7 @@ 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: Patched use of macOS 10.9 window occlusion API for macOS 10.8 compatibility
  - [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

@@ -37,6 +37,12 @@
 //       having been (according to documentation) added in Mac OS X 10.7
 #define NSWindowCollectionBehaviorFullScreenNone (1 << 9)
 
+// HACK: This enum value is only available on Mac OS X 10.9 onwards,
+//       so it needs to be defined on prior versions
+#if MAC_OS_X_VERSION_MIN_REQUIRED < 1090
+#define NSWindowOcclusionStateVisible (1UL << 1)
+#endif
+
 // Returns whether the cursor is in the content area of the specified window
 //
 static GLFWbool cursorInContentArea(_GLFWwindow* window)
@@ -311,7 +317,11 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
 
 - (void)windowDidChangeOcclusionState:(NSNotification* )notification
 {
-    if ([window->ns.object occlusionState] & NSWindowOcclusionStateVisible)
+    if (
+#if MAC_OS_X_VERSION_MIN_REQUIRED < 1090
+        [window->ns.object respondsToSelector:@selector(occlusionState)] && (NSUInteger)
+#endif
+        [window->ns.object occlusionState] & NSWindowOcclusionStateVisible)
         window->ns.occluded = GLFW_FALSE;
     else
         window->ns.occluded = GLFW_TRUE;