Allow building with -fno-common, aka support for GCC 10
This series allows building with GCC 10, which enforces -fno-common.
In the process of doing so, it promotes the optional wayland-client 1.10 to mandatory.
In practise if a distribution is using GCC 10, they are bound to have newer wayland. Although I've not checked all distros, Debian stable/buster is using 1.16 - a nice indication.
Note: I've based this on top of the xdg-shell MR and the two are fairly connected.