New include guard in seq_event.h breaks DSSI header

[GitHub issues - opened <github@alsa-project.org>]

alsa-project/alsa-lib issue #455 was opened from cannam:

Hello! I am one of the original authors of [DSSI](https://dssi.sourceforge.net/), a soft-synth API that was created about 20 years ago. It's not an active project, it was basically "done" by about 2011 but is still used in a number of applications.

The DSSI header `dssi.h` includes `<alsa/seq_event.h>` in order to pick up a structure for MIDI-like events.

I see that following a report of breakage in #431, a guard was added to seq_event.h in ea8972c to warn against including that header directly.

The official fix seems to be to include the whole of `<alsa/asoundlib.h>`, and that is what packagers appear to be patching the DSSI header with currently (and I see it is also what the guard does itself after emitting the warning). Unfortunately from the perspective of DSSI as an API, that isn't really a fix at all because all we need is an event structure - there is no other dependency on ALSA as a library and there was never any intention to require linking with ALSA.

Probably the historical error was to use an ALSA header at all, and the best option for us might be to duplicate enough of the contents of `seq_event.h` locally to sever the dependency that way.

But I thought I should first check whether you can suggest a better approach, for example whether you might be open to making it possible (again) to make use of `seq_event.h` specifically without pulling in a dependency on everything else in ALSA.

Thank you!

Issue URL     : https://github.com/alsa-project/alsa-lib/issues/455
Repository URL: https://github.com/alsa-project/alsa-lib