build: real clients build disabling

The weston-info and weston-simple-im couldn't be build is the toytoolkit is not availlable
build: check the libxkbcommon availlability (auto)
keyboard: if xkbcommon isn't availlable, informations about keymap is useless

This patch is allowed if xkbcommon header files are not present.
wayland-backend: doesn't handle keymap if libxkbcommon is not availlable
build: remove build warning
build: does the clients independent from xkbcommon when it's possible or doesn't build them