Peter M. Groen / libmodbus

It's strongly recommended to update your libmodbus library if you
use it in a slave/server application in a not trusted environment.

Debian package of libmodbus 3.0.4 already contains a patch to
mitigate the exploit but the patch isn't as strong than this one.

Related to https://github.com/stephane/libmodbus/issues/83

Thank you very much James Nutaro.

Thanks to Jan Kardell.

This regression comes from Visual Studio patches.

- change return argument from void to int
- update documentation

Thanks to Karl Palsson.

Enable out-of-source build

Add top build directory to tests. Otherwise modbus-version.h
won't be found during the build process.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>