When we selected ncursesw, look for tinfow as well.  Since ncurses-6.1
the binary compatibility between tinfo and tinfow disappeared, resulting
in crashes and other odd behaviour.

This change checks for tinfo or tinfow based on which ncurses we found
to continue with.

https://bugs.gentoo.org/651552

[Thanks to Fabian Groffen for the patch.  I made a minor modification
to fall back to tinfo because pre-6.1 systems may not have a tinfow
but still need tinfo to compile.]