Fix ncurses to not add /usr/lib to rpath.

[mat]

Summary

There are three cases here:

1. USES=ncurses or USES=ncurses:base and devel/ncurses is not present: use base, no need to add rpath as there is only one libncurses.so present, in /usr/lib
2. USES=ncurses and devel/ncurses is present or USES=ncurses:port: use ports, and add rpath to LOCALBASE/lib so that the right one is picked
3. USES=ncurses:base and devel/ncurses is present fail (it would warrant using -rpath /usr/lib, but it error's out during the sanity stage)