Fix wxOSX build with < 10.9 SDK after r77511.

We need to guard the use of 10.9+ symbols such as NSActivityOptions at
compile-time too and not only at run-time.

See #16413, closes #16529.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77544 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Always disable "Back" wxWizard button for the starting page.

Do it even if the start page has a previous page (e.g. because the wizard has
been started in the middle for some reason).

Closes #16535.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77545 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775