I updated the macbuikld list, on the opencpn.org page GTK isn't needed.
The Cmake script used the wx-config that its first in your path to find the installed wxWidgets,
if you compile an own version of wvWidgets, make sure you add it's binary directory first to you path environment
they run cmake and it will be used. There is no need to change anything permanent.
env PATH=/Users/someone/xwwidgets/bin/:$PATH cmake ..