Reuse native curses(8) library on NetBSD

Summary:
The reason for it is limit of detecting ncurses on various systems. For
example, Ubuntu ships with <curses.h> and linkage from <ncurses.h>, <ncurses.h>
isn't detected by CMake. Detecting `<curses.h>` on NetBSD is reusing
conflicting header from the host curses(8) and pkgsrc's ncurses library.

ncurses ships on most (till conflicting) systems with curses.h. On NetBSD it
might be conflicting, so the ncurses headers are installed with pkgsrc to a
subdirectory "ncurses/".

Patch by Kamil Rytarowski. Thanks!

Reviewers: clayborg

Subscribers: youri, akat1, brucem, joerg, lldb-commits

Differential Revision: http://reviews.llvm.org/D14037

llvm-svn: 252250