Add wxHAS_3STATE_CHECKBOX symbol.

Add wxHAS_3STATE_CHECKBOX symbol.

This symbol is defined for the ports that support wxCHK_3STATE style. While
most of the ports do support it, a couple still do not and having this symbol
makes it more convenient to exclude 3-state-checkbox-specific code, like in
CheckBoxTestCase.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65825 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775