Add compilation guards for macOS 10.10 to avoid warnings

"[NSColor linkColor]" gives a compile-time warning even if we don't use
it during the run-time, so check MAC_OS_X_VERSION_MAX_ALLOWED before
using it.

src/osx/cocoa/settings.mm

@@ -163,11 +163,13 @@ wxColour wxSystemSettingsNative::GetColour(wxSystemColour index)
         sysColor = [NSColor windowBackgroundColor];
         break;
     case wxSYS_COLOUR_HOTLIGHT:
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_10
         if ( WX_IS_MACOS_AVAILABLE(10, 10) )
         {
             sysColor = [NSColor linkColor];
         }
         else
+#endif
         {
             // OSX doesn't change color on mouse hover
             sysColor = [NSColor controlTextColor];