Upgrade to GCC 7

This upgrades to a modern GCC 7 toolchain in build-essential.

Lots of new issues are caused by the upgrade, mostly due to components that build with -Werror enabled. Where possible I have upgraded these to the latest versions, which often fix the compile warnings. Some components haven't fixed the issues upstream and one (SYSLINUX) can't be upgraded, so I have patched those.

The GNOME system is now a bit inconsistent as it has a mix of GNOME 3.18, 3.22 and 3.24 components. I don't have time to do a full upgrade and fix the existing issues there right now. I propose we wait until we can use BuildStream and bst track which makes it a whole lot more convenient to keep large sets of components up to date.

Fixes #2 (closed)