xbps-src: avoid mkdir {build,dest}dir in /.

xbps-src/shutils/init_funcs.sh.in

@@ -63,7 +63,11 @@ set_defvars()
 
 	for i in DESTDIR PACKAGESDIR BUILDDIR SRCDISTDIR; do
 		eval val="\$XBPS_$i"
-		[ ! -d "$val" ] && mkdir -p $val
+		if [ ! -d "$val" ]; then
+			mdir=$(dirname $XBPS_MASTERDIR)
+			[ -z "$IN_CHROOT" -a "$mdir" = "/" ] && continue
+			mkdir -p $val
+		fi
 	done
 
 	xver=$(xbps-bin.static -V|awk '{print $2}')