Also drop curses libs from libs_softmmu.  Add CURSES_{CFLAGS,LIBS}
variables so we can use them for linking the curses module.

Also make target/unicore32/helper.o depend on curses which uses curses
directly for some reason ...

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20180301100547.18962-12-kraxel@redhat.com