Use __DARWIN__ instead of __WXOSX__ in non-GUI code.

This fixes compilation of non-wxOSX ports (e.g. wxGTK) under OS X.

Also make the difference between the two symbols more clear in the
documentation.

Closes #14503.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72180 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/evtloopsrc.h

@@ -89,9 +89,11 @@ inline wxEventLoopSource::~wxEventLoopSource() { }
 
 #if defined(__WXGTK20__)
     #include "wx/gtk/evtloopsrc.h"
-#elif defined(__WXOSX__)
+#endif
+
+#if defined(__DARWIN__)
     #include "wx/osx/evtloopsrc.h"
-#endif // platform
+#endif
 
 #endif // wxUSE_EVENTLOOP_SOURCE