Almost all is sorted. I am running Debian 8 Jessie with an old kernel because of my display card.
Athough the 'missing' file is
QtGui/Qaction
it is actually found at
QtWidgets/QAction
It changed its location between qt4 and qt5. Apparently the Makefile allows for this, but quite how is beyond my paygrade. Could this cause a problem further down the line? The file comes from
qtbase5-dev
which I thought I had loaded by copying your instructions for building from source.
Anyway, I now have a copy of the source code, an installation of qt-designer from a Debian package and an installation of the qt5 development suite from Qt. I can build LibreCad with either Qt package or from the command line.(3 routes). When I tred to run LibreCAD I obtained a series of messages like
Plugin verification data mismatch in '/usr/lib/librecad/libalign.so
Plugin verification data mismatch in '/usr/lib/librecad/libasciifile.so'
Plugin verification data mismatch in '/usr/lib/librecad/libimportshp.so'
.......
These came from an earlier version of LibreCAD which I had installed as a Debian package. I have now removed that package and all is well. Does this mean that anyone developing the package cannot simultaneously have a version installed as a package?