I found that on macOS, setting the libncurses.a and libtinfo.a into the
same CURSES_LIBRARY variable does not find or link with libtinfo.a.
To fix this, this commit adds a separate TINFO_LIBRARY variable.
In the end, CURSES_LIBRARIES and the Curses::curses CMake TARGET will
still have both libraries set, if both are provided.
This fix is necessary if the ncurses was built with `--with-terminfo`.
I think we got away without it on Linux because of pkg-config.
I also found that Apple's ncurses is prioritized by PkgConfig over one
specified by using variables. To this end, we'll skip PkgConfig if
the include path was provided.