Allow to build without man pages in case the user doesn't want that

autogen.sh

@@ -1,5 +1,15 @@
 #!/bin/sh
 set -ex
-make -C man -f mans.mk nbd-server.1.sh.in nbd-server.5.sh.in nbd-client.8.sh.in nbd-trdump.1.sh.in nbd-trplay.1.sh.in nbdtab.5.sh.in
+if [ -z "$(type -p docbook2man)" ]; then
+	if [ -z "$NO_MANPAGES" ]; then
+		echo "E: docbook2man not found. Set NO_MANPAGES to a nonzero value to build without man pages"
+		exit 1
+	fi
+	echo "W: docbook2man not found. You cannot distribute a tarball from this build, but we'll allow you to build without manpages."
+	cd man
+	touch nbd-server.1.sh.in nbd-server.5.sh.in nbd-client.8.sh.in nbd-trdump.1.sh.in nbd-trplay.1.sh.in nbdtab.5.sh.in
+else
+	make -C man -f mans.mk nbd-server.1.sh.in nbd-server.5.sh.in nbd-client.8.sh.in nbd-trdump.1.sh.in nbd-trplay.1.sh.in nbdtab.5.sh.in
+fi
 make -C systemd -f Makefile.am [email protected]
 exec autoreconf -f -i