This is done using the new QT_TAG_FILE and QT_DOC_URL cmake options.
QT_TAG_FILE should be set to the path of the qt.tags tagfile. In ubuntu
this file is provided by the qt4-doc-html package, and is located at
/usr/share/qt4/doc/html/qt.tags
QT_DOC_URL should be currently set to http://doc.qt.io/qt-4.8/