Find ncurses from pkg-config if possible, and prefer separate libtinfo

Recent ncurses can be configured --with-termlib, which splits out the
terminfo-level functions from libncurses into a separate libtinfo.
This allows us to avoid an unnecessary dependency on libncurses.  (We
already avoided this on distributions that link with -Wl,--as-needed.)

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Closes #128 github pull request.