I had been indecisive about whether the definitions and calls of
store_cutbuffer and retrieve_cutbuffer should be compiled out
completely in GTK-without-X mode, or whether the definitions should be
left in as stubs and the calls still present. retrieve_cutbuffer ended
up with a definition but no call in that mode.
It was only an unused-function warning, but -Werror promoted it to an
error. Fixed by making up my mind: now the functions are completely
absent, and so are the calls to them.