Warn if AppDir needs too new glibc

Throw at least a warning if the glibc version needed to run is higher than the one shipped by the currently oldest still-supported Ubuntu LTS release.

Possibly require user to pass in `--accept-too-new-dependencies` or something along those lines in order to produce an AppImage if the glibc is "too new".

Depends on: https://github.com/AppImage/libelfutil/issues/6