The official, signed AppImage will be build locally for now for
security of my key, but I still want it to be built in a clean,
controlled environment based off a suitably old Linux distribution for
maximal portability in the AppImage.
We were putting the value of LT_CURRENT in the versioned symbol map
instead of the soname major version, which was creating binary
incompatibility where none was present. Unfortunately, this bug is
already out there for qpdf 7.0.0, so we'll have to work around it for
now.
