MAVlink 2.0 compatibility issues

Hi,
it seems there is a problem with MAVlink v2.0, files generated using pymavlink in C using ardupilotmega msg definitions.

```
mavlink_json.o: In function `mavlink_message_send_args':
/APWeb/mavlink_json.c:271: undefined reference to `mavlink_set_proto_version'
/APWeb/mavlink_json.c:273: undefined reference to `mavlink_get_crc_extra'
collect2: error: ld returned 1 exit status
Makefile:18: recipe for target 'web_server' failed
make: *** [web_server] Error 1
```

Start by inspecting the Makefile target for web_server and the generated mavlink_json.c references around lines 271-273, then reproduce the linker failure with the ardupilotmega definitions. Done means the MAVLink 2.0-generated C sources link successfully and the web_server target completes without these undefined references.

Written by the indexing model from the issue text.