SR_OK: a match was found.
SR_ERR: no match.
SR_ERR_DATA: a match was found but the module cannot handle the input.
SR_OK_CONTINUE: some module didn't have enough data to be sure.
This returned an array of structs with an NULL-ed element at the end.
The drivers still do this, but the wrappers now make and free a NULL-
terminated array around it.
sr_output_options_free() now takes the pointer returned by
sr_output_options_get(), instead of the module owning it.
This is intended for setting (or getting) the amplitude of a source
which doesn't really have an MQ associated with it, such as the demo
driver's analog channels.
This is needed so that the C++ bindings, the header for which
references "libsigrok/libsigrok.h", can have a valid include
directory passed to build them before the headers are installed.