Previously, ncurses.mk tests for header instead of the shared library
symlink itself, and this would cause problem for portsnap builder
(addressed differently by including the header file).

Make the test more consistent with port version of ncurses by checking
the shared library instead. (the location is the same on all supported
versions of FreeBSD).

Approved by: portmgr (bapt)
Differential Revision: https://reviews.freebsd.org/D3326