Allow linking against qr-code-generator system library
In https://gitlab.gnome.org/GNOME/network-manager-applet/merge_requests/45 support for displaying a QR Code for hotspot configs was added using https://github.com/nayuki/QR-Code-generator as internal copy of that library. In distros like Debian, we try to avoid using embedded code copies and since that library is already packaged (see https://tracker.debian.org/pkg/qr-code-generator), it would be nice if NetworkManager provided an option to link against the system version instead of the embedded one. I assume you will still want to keep the embedded copy for distros not having packaged that library yet.
See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=910124#42