put ncurses auxiliary libs in PREFIX/gnu/lib

Keep ncurses in PREFIX/lib, but place lib{menu,panel,form} in
PREFIX/gnu/lib; this way other programs will be able to link with
-lncurses without having to specify rpath (unless they also link with
those other libraries, but it seems to me not many programs do).
don't fail the build without DEPENDS_IPS
build mutt without rpath (and it works this time :)