Use FindCurses.cmake as fallback to find curses libraries

In some systems (e.g. Suse 13.1 x86_64 here) looking for curses/ncurses using check_library_exists() might fail even when it is installed. For recent versions of cmake there is already a FindCurses(find_package) module that can be reused for this purpose.

- I have not tested in windows but I don't think this module works for MinGW (issue #69)
- The commit only uses FindCurses if no other option works, so it should not break the check for systems that don't have it