Autotroll (http://tsunanet.net/autotroll/) allows configure to
determine the right compile flags needed to access Qt header files
and libraries.
SyncEvolution backends can append new Qt modules to the
"need_qt_modules" variable in the configure stub. If set, configure
will call AT_WITH_QT().
Excluding the GUI part of Qt is currently hard-coded. Enable it again
with "need_qt_modules=$need_qt_modules +gui" if necessary.