cocoa_gl_ctx: gate CGDisplayModeRef refresh-rate read on 10.6 SDK

GCC 4.0 (Xcode 3.1, PowerPC) reports:

    cocoa_gl_ctx.m:251: error: 'CGDisplayModeRef' undeclared
        (first use in this function)

CGDisplayModeRef, CGDisplayCopyDisplayMode, and CGDisplayModeGet-
RefreshRate all arrived in 10.6 Snow Leopard.  The 10.5 Leopard
SDK only exposes the older CFDictionaryRef-based CGDisplayCurrent-
Mode API, which is a different enough shape that it's not worth
wiring up for a single refresh-rate readout.

Gate the existing OSX block on MAC_OS_X_VERSION_10_6 (defined only
on 10.6+ SDKs).  On older SDKs, return 60.0f — same pattern the
iOS #else branch already uses for targets older than iOS 10.3 /
tvOS 10.2 that don't have maximumFramesPerSecond.

Related: 667256a gated the same CGDisplayModeRef family in
dispserv_apple.m behind a RARCH_HAS_CGDISPLAYMODE_API macro.  This
change uses the raw SDK gate (MAC_OS_X_VERSION_10_6) to stay
consistent with the other MAC_OS_X_VERSION_* gates already in this
file at lines 206, 298, 375, 396, 402, 610.

Modern macOS builds are unaffected.

Author: LibretroAdmin

gfx/drivers_context/cocoa_gl_ctx.m

@@ -247,11 +247,19 @@ static void cocoa_gl_gfx_ctx_get_video_size(void *data,
 static float cocoa_gl_gfx_ctx_get_refresh_rate(void *data)
 {
 #ifdef OSX
+#ifdef MAC_OS_X_VERSION_10_6
+    /* CGDisplayModeRef and CGDisplayCopyDisplayMode are 10.6+.
+     * On the 10.5 Leopard SDK only the older CFDictionaryRef-based
+     * CGDisplayCurrentMode API exists; not worth wiring up for a
+     * refresh-rate readout, so fall back to 60. */
     CGDirectDisplayID mainDisplayID = CGMainDisplayID();
     CGDisplayModeRef currentMode = CGDisplayCopyDisplayMode(mainDisplayID);
     float currentRate = CGDisplayModeGetRefreshRate(currentMode);
     CFRelease(currentMode);
     return currentRate;
+#else
+    return 60.0f;
+#endif
 #else
     if (@available(iOS 10.3, tvOS 10.2, *))
        return [UIScreen mainScreen].maximumFramesPerSecond;