libtool: fix help2man related failure

Libtool sometimes requires help2man to build manpages, but since we
don't use/need them and want to avoid pulling in and/or building
dependencies for that we just disable it. Fixes:
http://autobuild.buildroot.net/results/bfb/bfb9e298196d877b623580f882b6fe74b30bec9b/

Also add host-m4 as dependency, it's not strictly required but silences
many build warnings.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libtool/libtool.mk

@@ -8,6 +8,8 @@ LIBTOOL_VERSION = 2.4.4
 LIBTOOL_SOURCE = libtool-$(LIBTOOL_VERSION).tar.xz
 LIBTOOL_SITE = $(BR2_GNU_MIRROR)/libtool
 LIBTOOL_INSTALL_STAGING = YES
+LIBTOOL_CONF_ENV = HELP2MAN=true
+LIBTOOL_DEPENDENCIES = host-m4
 LIBTOOL_LICENSE = GPLv2+
 LIBTOOL_LICENSE_FILES = COPYING