link libreadline with the new libtermcap (filter) library
illumos issue 1168 introduces a new libtermcap "fiter" library
that allows libreadline to pull just what it needs from libcurses,
and do so in a way that does not bind consumers of libreadline
to any particular curses library.
In order to avoid the need to add -ltermcap or -lcurses to all
applications that link with libreadline, we should build our
libreadline to have a dependency on this new filter library.
Note that fixing 1168 is a prerequisite for this issue.