Commit a021c500edf9939eace2c7bbae6b9290b430f7be removed the qt.tag copy
in the qca sources, so don't point the Doxyfile to it.
Its usage was not optimal anyway, since the references to Qt classes
where still considered within the qca apidox.
Do not use doxygen directly. Often cmake build out-of-source.
So it usefult to have doc make target. It is required to generate
documentation for generating header file (qca_version.h) which places
in build tree.
