BUG: fix qt5 compilation on ubuntu 18.04
fixed bug #1687 (closed) . We should perhaps backport to branch 6.6?
This commit not only includes the bugfix for qt5 on ubuntu but also some improvments in the qt5 external project.
The fix concern only the following modification:
- add -qt-harfbuzz in the configure script to fix libz conflict
Other minor updates:
- remove commented cmake configuration related to qt4