Makefile: use "mkdir -p" instead of "-make"

For some casual readers of the Makefile, "mkdir -p" is
probably a little easier to read than the equivalent "-make".

Reported-by: Reuben Thomas <rrt@sc3d.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

Makefile

@@ -17,7 +17,7 @@ allgz: gz all
 allbz: bz2 all
 
 screen:
-	-mkdir not_installed
+	mkdir -p not_installed
 	for i in man?/*; do \
 		if [ $(MANDIR)/"$$i" -nt "$$i" ]; then \
 			cmp -s $(MANDIR)/"$$i" "$$i" > /dev/null 2>&1; \