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>
Merge pull request #106 from yegorich/oos-build

Enable out-of-source build
Remove unsupported -Wtype-limits for GCC < 4.3.5 (closes #109)