Utf8 fix

This commit incorporates (cherry-picking) [@forslund changes](https://github.com/cgdb/cgdb/pull/27) to add UTF-8 support by linking to ncursesw. It also removes one commit related to warning messages from his pull-request, since I haven't experienced such warnings, and see no reason for them to show up because of ncursesw usage.

It extends forslund changes for UTF-8 support by replacing use of `isprint` by `!iscntrl` for non-printable character's exclusion, which allows a wider range of characters to be displayed, including utf8 characters, in other windows beyond the source window.