graphics/xpdf4: add Qt5/Qt6 flavors

[osa]

Summary

Add Qt5/Qt6 flavors to make available graphics/xpdf4 application for Qt6 users

[osa]

With these changes it looks better now:

% make -C graphics/xpdf4 -VPKGNAME FLAVOR=qt5
xpdf4-4.05_3,1
% make -C graphics/xpdf4 -VPKGNAME FLAVOR=qt6
xpdf4-qt6-4.05_3,1
% make -C graphics/xpdf -VPKGNAME FLAVOR=qt5 
xpdf-4.05_3,1
% make -C graphics/xpdf -VPKGNAME FLAVOR=qt6
xpdf-qt6-4.05_3,1

[fuz]

graphics/xpdf should be a meta port or just install symlinks imho. I don't like this whole “build the same thing twice under a different name” business.

[cy]

This looks better.