Begin phasing out support for Qt 4?

[John W. Eaton]

By default, the configure script currently checks for Qt5 and then Qt4 
if Qt5 doesn't appear to be working.  This seems to cause a bit of 
trouble and confusion when people have both versions installed, think 
they will be using Qt5 but some missing features cause Octave's build to 
revert to using Qt4 instead.

I checked in the following changeset to provide more info in the 
configure summary about the Qt version that is found, and any missing 
modules in case Qt is not found or is disabled.

  http://hg.savannah.gnu.org/hgweb/octave/rev/cc21f66ecb1c

Maybe this change will help with diagnosing Qt configuration problems.

Is Qt5 required now for a fully functional build?  It seems so, as the 
new off-screen printing and help features rely on Qt5 features, correct? 
 If so, maybe it is time to check only for Qt5 by default, and not 
check for Qt4 unless it is explicitly requested on the configure command 
line?

jwe