Add libmodbus version checks

libmodbus version 3.1.2 introduced backwards incompatible API changes in
modbus_set_response_timeout and modbus_set_byte_timeout functions. This
patch adds conditional compilation to use the API appropriately based
upon the result of the LIBMODBUS_VERSION_CHECK macro.

Issue: https://github.com/LinuxCNC/linuxcnc/issues/105


Signed-off-by: Dale Lukas Peterson <hazelnusse@gmail.com>