I would need to compile the Arduplane firmware, v3.8.0 using mavlink v1.0 instead of v2.0.
As far as I understood, the messages generation (header files) is made automatically during the compilation.
What parameter should I set in order to do that?
The standard FW already supports both mavlink 1.0 and 2.0. No need to recompile anything. But you do need to tell MavProxy and/or Mission planner that you only want to use mavlink 1.0.
The ArduPilot boots in mavlink 1.0 mode, the ground station is the one the issues a request to change to 2.0. If the GCS does not do that, Ardupilot remains in 1.0 mode.