Fix configure script generated by pkgconf-2.3.0

[vbraun]

Pkgconf 2.3.0 changes the PKG_PROG_PKG_CONFIG macro to error out instead of marking it as not found. Arguably this is the correct default for most but not for us, since we support building on macos where Apple, in their infinite wisdom, decided not to ship their dev tools with it.

See also: https://github.com/pkgconf/pkgconf/blob/master/NEWS

dnl PKG_PROG_PKG_CONFIG([MIN-VERSION], [ACTION-IF-NOT-FOUND])
dnl ---------------------------------------------------------
dnl Since: 0.16
dnl
dnl Search for the pkg-config tool and set the PKG_CONFIG variable to
dnl first found in the path. Checks that the version of pkg-config found
dnl is at least MIN-VERSION. If MIN-VERSION is not specified, 0.9.0 is
dnl used since that's the first version where most current features of
dnl pkg-config existed.
dnl
dnl If pkg-config is not found or older than specified, it will result
dnl in an empty PKG_CONFIG variable. To avoid widespread issues with
dnl scripts not checking it, ACTION-IF-NOT-FOUND defaults to aborting.
dnl You can specify [PKG_CONFIG=false] as an action instead, which would
dnl result in pkg-config tests failing, but no bogus error messages.

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:hourglass: Dependencies

[github-actions[bot]]

Documentation preview for this PR (built with commit f882056dab82ba5c59713c70b08ca53b3df83092; changes) is ready! :tada: This preview will update shortly after each push to this PR.

[kwankyu]

I am new to this territory. So perhaps a stupid question: Is pkgconf or pkg-config a build prerequisite on mac? I think it should because either of them must provide the macro PKG_PROG_PKG_CONFIG in the first place. But then why it does not appear here: https://doc-release--sagemath.netlify.app/html/en/reference/spkg/_prereq ?

[vbraun]

The pkgconf m4 scripts are, together with autoconf/autotools, a prerequisite for generating configure.ac -> configure.

But they are not required for building Sage from source. The bootstrap script will either run autoconf or, failing that, download a pre-generated configure. This is build/pkgs/configure, and I generate a new one whenever I push a new version.

Fedora 40 -> 41 updated the pkgconf m4 scripts, so this the breakage comes from.