modularize `aclocal.m4`?

[Werner LEMBERG]

In standard GNU projects, the `aclocal.m4` file gets auto-generated
with the `aclocal` script; the necessary M4 scripts are either taken
from the system or from a directory that gets specified with some
command line options to `aclocal`.

Looking into the git history of LilyPond's `aclocal.m4` file I can see
that this was true in the very beginning.  However, later on people
started to modify this file manually.  Was this intentional?

I suggest to use `aclocal` again, modularizing the current stuff in
`aclocal.m4` by splitting it into various `.m4` files in a `m4`
top-level subdirectory.

In another mail I wrote that the current problems with not showing
help for pkg-config environment variables is due to `true` statements
in calls to PKG_XXX macros.  This was a wrong guess.  Instead,
`aclocal.m4` contains a very old version of `PKG_CHECK_MODULES` (from
2004!) that was much improved meanwhile.[*] Had we a modular design it
would be very simple to just update a local copy of `pkg.m4`, and we
would immediately benefit from the improvements.  Ditto for other
stuff too, I guess.


    Werner


[*] See issue #5830 for a fix.