Add wxUSE_MARKUP and wrap SetLabelMarkup() in it.

Make it possible to disable all the new markup-related code by setting
wxUSE_MARKUP to 0 in setup.h or using configure --disable-markup option.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67062 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/doxygen/mainpages/const_wxusedef.h

@@ -170,6 +170,7 @@ library:
 @itemdef{wxUSE_LONGLONG, Use wxLongLong class.}
 @itemdef{wxUSE_LONGLONG_NATIVE, Use native <tt>long long</tt> type in wxLongLong implementation.}
 @itemdef{wxUSE_LONGLONG_WX, Use generic wxLongLong implementation.}
+@itemdef{wxUSE_MARKUP, Provide wxControl::SetLabelMarkup() method.}
 @itemdef{wxUSE_MDI, Use wxMDIParentFrame, and wxMDIChildFrame}
 @itemdef{wxUSE_MDI_ARCHITECTURE, Use MDI-based document-view classes.}
 @itemdef{wxUSE_MEDIACTRL, Use wxMediaCtrl.}