Branch/topic/default/fix explicit with ncurses

An earlier change (12cc6e4e8edf), which effectively made ncurses used by default, was in a sense backwards-incompatible, in that if one actually included --with-ncurses, the result would not build. The patch, perhaps somewhat inelegantly, fixes that.

Note, not tested without the switch, i.e. to see if the defaulting would still work...