Fix warnings about wxOSX_USE_CARBON being undefined when using g++.

Test for __WXMAC__ before testing wxOSX_USE_CARBON to avoid -Wundef warnings
from g++ in non-Mac builds.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69367 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/popupwin.h

@@ -134,7 +134,7 @@ protected:
 
     // wxOSX/Carbon doesn't generate mouse capture loss events currently so
     // poll for the capture loss ourselves.
-#if wxOSX_USE_CARBON
+#if defined(__WXMAC__) && wxOSX_USE_CARBON
     // check if the mouse needs captured or released
     void OnIdle(wxIdleEvent& event);
 #endif