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?

[matthiasclasen]

thats a good idea

[bbhtt]

IMO we can produce a large warning in Meson when it is missing at build time and ask nicely to add these as runtime dependencies.

Same for any other external binary that flatpak-builder shells too.

Ultimately, these build time checks do not guarantee that people are adding them to runtime dependencies in their packages.

I'm not a fan of adding runtime dependencies as mandatory build time dependencies in Meson.