Make it clearer that elfutils is required for building most flatpaks via flatpak-builder

[Cogitri]

As can be seen with #92 and #261 lots of distros miss the implicit elfutils dependency of flatpak-builder. Void and Alpine Linux still don't have it (I'll add it in a bit). Maybe it'd be a good idea to point this out in the configure script or to at least mention it in the readme?