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.