devel/ncurses: Prevent configure from touching LOCALBASE

Synth detected that devel/ncurses was touching $LOCALBASE/lib during
the build, and narrowing it down, it turned out to be configure:

Modified files/directories:
     usr/local/lib [ modification ]

Throught manual inspect, I tracked this down to the long names support
test. Caching the conf value to "yes" results in all phases between
configure and stage (inclusive) not touch base.

Approved by: just fix it