Add waffle 1.7.1 candidate

In a gist:

• cmake: pass deps' cflags to the build
• gbm: fix crash when platform lacks modifier support
• gitlab-ci: stabilise CI runs under X
• gitlab-ci: add more warnings, make all warnings fatal
• gitlab-ci: update to bullseye
• meson: add override_* support, when using waffle as submodule
• meson: skip installing bash completion when custom prefix is used
• meson: silence deprecation warnings
• meson: generate cmake files only on Windows
• misc: zsh completion
• misc: fix dozens of compiler warnings
• misc: update website references
• wayland: fix build against wayland 1.20