Fix test for __WXOSX__ in common code.

Check whether it's defined, not whether it's true.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76941 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Don't consider hidden root item to be visible in wxGenericTreeCtrl.

This fixes wrong results returned by GetFirstVisibleItem() in the tree control
with wxTR_HIDE_ROOT style.

Closes #4482.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76942 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775