collectd is currently the only user of libmodbus.
Thus include the version of libmodbus which allows collectd
modbus plugin to build.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
I used the current version found in package feed, upgraded to the latest
stable upstream version, fixed the download url and added me as maintainer.
Signed-off-by: Michael Heimpold <mhei@heimpold.de>