build: add WITHOUT_CLEAN workaround for 821aa63a0940

The *w variants of ncurses directories went away, and the remaining names
build the widechar variants instead of non-widechar variants. As such, the
entire ncurses tree should be regenerated.

Key off of lib/ncurses/ncursesw being present and remove the whole ncurses
hierarchy if it is.

Reviewed by: emaste (IRC)