Bring back Qt5 support for signond client library
This reintroduces libsignon-qt5-1 and libsignon-qt5-dev binary packages by dually building signond and scraping the library / include files out of the Qt5 DESTDIR.
This legacy support of libsignon-qt5-1 and libsignon-qt5-dev is essential to the Lomiri Online Accounts stack in Lomiri. Without it we won't be able to ship lomiri-calendar-app and lomiri-addressbook-app in Debian 13.
The Qt6 migration of Lomiri will take more time, so please support Lomiri in Debian by accepting this MR. Thanks!!!
(Also, let me know if / what is needed to provide a proper debian/libsignon-qt5-1.symbols file. One option could be to not ship that file for the Qt5 variant of the library).
Edited by Mike Gabriel