doc.pri requires QT.xxx.variable variables but they are not set
the first (and normally only) time qmake parses doc.pri. As a
result, you can't build docs without first manually running
"make qmake".

This change adds an explicit dependency such that all of the doc
commands (docs, online_docs, qch_docs, dita_docs) first force
qmake to run and process doc.pri again. This ensures that the
QT.xxx.variable variables have the appropriate values.

Change-Id: I688c432284605e9f56e7597f564f1c50e972f059
Reviewed-on: http://codereview.qt-project.org/5779


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Alex <alex.blasche@nokia.com>