Install modbus lib per build-target

By using --exec_prefix=${RELEASE_DIR}/${BUILD_TARGET} which seems to be the standard way for target-specific install via autotools, the install directory would look like:

modbus/<version>/share
modbus/<version>/include
modbus/<version>/x86_64-linux/lib/libmodbus.a
modbus/<version>/x86_64-yocto-linux/lib/libmodbus.a

@m.marn, fine for you like this ? If you agree, I would modify the install folder structures for snap7 accordingly. (And I suppose we could have the same for silecs-communication ... no idea what's the standard target-specific folder structure for cmake, though I bet it's the same 🙂 )