Fix https://bugzilla.libsdl.org/show_bug.cgi?id=5306

(Also see: https://bugzilla.libsdl.org/show_bug.cgi?id=4822)

Building the current tree against 10.8 SDK, clang emits the following warning:
src/video/cocoa/SDL_cocoawindow.m:1846:27: warning: instance method '-isOperatingSystemAtLeastVersion:' not found (return type defaults to 'id') [-Wobjc-method-access]
            ![processInfo isOperatingSystemAtLeastVersion:version]) {
                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/MacOSX10.8.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSProcessInfo.h:20:12: note: receiver is instance of class declared here
@interface NSProcessInfo : NSObject {
           ^
1 warning generated.

isOperatingSystemAtLeastVersion is an 10.10 thing.

src/video/cocoa/SDL_cocoawindow.m

@@ -57,6 +57,9 @@
 #ifndef MAC_OS_X_VERSION_10_12
 #define NSEventModifierFlagCapsLock NSAlphaShiftKeyMask
 #endif
+#ifndef NSAppKitVersionNumber10_14
+#define NSAppKitVersionNumber10_14 1671
+#endif
 
 @interface SDLWindow : NSWindow <NSDraggingDestination>
 /* These are needed for borderless/fullscreen windows */
@@ -1833,17 +1836,7 @@ Cocoa_SetWindowFullscreen(_THIS, SDL_Window * window, SDL_VideoDisplay * display
         /* Hack to fix origin on Mac OS X 10.4
            This is no longer needed as of Mac OS X 10.15, according to bug 4822.
          */
-        NSProcessInfo *processInfo = [NSProcessInfo processInfo];
-#if MAC_OS_X_VERSION_MAX_ALLOWED < 101000 /* NSOperatingSystemVersion added in the 10.10 SDK */
-        typedef struct {
-            NSInteger majorVersion;
-            NSInteger minorVersion;
-            NSInteger patchVersion;
-        } NSOperatingSystemVersion;
-#endif
-        NSOperatingSystemVersion version = { 10, 15, 0 };
-        if (![processInfo respondsToSelector:@selector(isOperatingSystemAtLeastVersion:)] ||
-            ![processInfo isOperatingSystemAtLeastVersion:version]) {
+        if (floor(NSAppKitVersionNumber) <= NSAppKitVersionNumber10_14) {
             NSRect screenRect = [[nswindow screen] frame];
             if (screenRect.size.height >= 1.0f) {
                 rect.origin.y += (screenRect.size.height - rect.size.height);