Add required modules for Debian stable?

See bug 402901.

Basically, on Debian 9 (current stable) they get an error message when using --initial-setup that kdesrc-build doesn't know which packages to install for "linux". Which is accurate enough, since we haven't filled in the list for debian jessie yet.

However looking into this a bit further, debian jessie has only Qt 5.7, and we still don't officially support also building Qt in kdesrc-build after they switched to using submodules. I have experimental build-include files that can get through most of Qt but would need more work (and potentially code changes in kdesrc-build) to call production-quality.

See my question here is: Should we even be supporting stable/LTS distributions as a target for kdesrc-build? Or should we require a recent enough distro that it can at least provide a supported Qt?

@ngraham would appreciate your thoughts also.