wwWidgets consists of two libraries - the widget library and the plugin for it that makes it possible to use the widgets in Designer. If you have an old version of the widget library installed and you try to build the plugin, it will link against the stale version of the widget library because it comes first in your library path. So you can either delete the libwwwidgets.so (or equivalent) file from the lib subdir of your Qt installation or you can install the new version of the library before building the plugin. The widget library is the "widgets" subproject of wwWidgets and the plugin is the "plugin" subproject of wwWidgets. So I suggest you do (assuming you are in the top level directory of the bundle):
$ cd widgets; qmake && make; su
# make install
# exit
$ cd ../plugin; qmake && make; su
# make install
# exit