>  I guess I'll have to join the fluidsynth list and ask for that.
> Maybe they can also help me with the other issue where even a x86_64
> fluidsynth installed for monterey via homebrew doesn't work in
> earlier versions of macOS.

I'm not sure this would be the right place to ask. What they provide,
works. The issues that the homebrew team is trying to address with
their way to do things are probably not related to the source code, but
more related to how things work on new Macs.

> So, now I need to find a way to have in my system an universal
> fluidsynth~ dynamic lib.

This seems like an unnecessarily hard quest, since homebrew doesn't
ship fat.

Wouldn't it be more feasible to build it separately for separate archs?
Then you wouldn't have to compile fluidsynth and all its dependencies
just to get fat libraries. 

Then there is the issue with codesigning that I don't see completely
through yet.

It seems on arm64/Monterey, I automatically get signed
binaries (adhoc signed, equivalent to 'codesign -s -') when building
externals. However, when I apply the localdep script to to such an
external file, it cannot be loaded afterwards. Pd immediately quits as
soon as it loads the external. An already signed binary
cannot/shouldnot be modified afterwards. 

I figured I had to remove the signature and sign all libraries in one
go, the external itself, but also all dylib files that it depends on.

codesign --remove-signature fluidsynth~.pd_darwin *.dylib
codesign -s - fluidsynth~.pd_darwn *.dylib

I'm not sure how/if that would work at all, if you have fat builds.

> It seems it would be ideal to me to have a single fat binary with the
> externals and the libs all together!

Shipping it together seems practical, I agree. I don't think it matters
much whether it is separate files or fat bundles.

>  I also mean this for windows and linux, though I have to say I'm
> quite ignorant on Linux and don't know about the challenges and also
> I don't know if people succeeded in providing binaries with included
> dynamic libs for linux, please tell me if there are any issues.

I often find things are a bit easier on Linux, but that's what I got
accustomed to. In the case of the fluidsynth~ external, using the
dynamic libraries from my distro (Ubuntu 22.04) doesn't seem practical,
as libfluidsynth.so.3 links against virtually half the system. After
executing the localdep script, I end up with 53 *.so files. Most of
them aren't actually used when libfluidsynth is used for the pd
external. Creating a more stripped down build of libfluidsynth probably
would make sense here.