Compute a default for `BUILD_CursesDialog` by building a small test
project that uses curses.  Disable `ccmake` by default if it fails,
and do not search for Curses as part of the main build.  This avoids
creating FindCurses cache entries when we are not considering ccmake.

If `BUILD_CursesDialog` is enabled (e.g. by the user) then warn if
curses cannot be found.