Peter M. Groen / libmodbus

Thank you @pboettch for initial proposal.

The result of LIBMODBUS_VERSION_HEX is left-shifted by an extra
factor of 8 bits.

For example, current implementation for version 1.2.3 would be
encoded like 0x01020300 instead of 0x010203.

https://www.securecoding.cert.org/confluence/display/seccode/DCL37-C.+Do+not+declare+or+define+a+reserved+identifier
Not applied to libmodbus constants for now...

Thanks to Denis Davydov

- partial revert of 93ce13656103cbb459ebd630d055a008dbb059f3
- it was a bit confusing to always edit modbus.h.in
- the header inclusion still unchanged (<modbus.h>)