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>