Modify rules, control so that .deb with v2.7.1 can be built; libqglviewer...

Hi, I have no experience in building debian packages at all. I only needed debian package with libqglviewer 2.7.1 to debug crashes in yade: https://gitlab.com/yade-dev/docker-yade/issues/1

It looks like libqglviewer 2.7.1 requires qt version above 5.4, so I had to remove qt4 packaging. I have no idea if this is correct. But at least I have a libqglviewer 2.7.1 package, and I can investigate this crash in yade.

In this branch there is still upstream 2.6.3, I manually deleted 4 dirs and 7 files, then unpacked the libQGLViewer-2.7.1.tar.gz to build this package. So in this commit are only the modifications in debian/ directory.

If you want to prepare libqglviewer 2.7.1 package maybe this merge request will be useful for you. But still, we better wait until I fix this crash in yade first ;)

EDIT: this crash is fixed in https://gitlab.com/yade-dev/trunk/merge_requests/160