sagemathgh-38954: Fix configure script generated by pkgconf-2.3.0

    
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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URL: sagemath#38954
Reported by: Volker Braun
Reviewer(s): Kwankyu Lee

configure.ac

@@ -222,7 +222,7 @@ dnl Exit autoconf with exit code 16 in this case. This will be
 dnl caught by the bootstrap script.
 m4_exit(16)])
 
-PKG_PROG_PKG_CONFIG([0.29])
+PKG_PROG_PKG_CONFIG([0.29], [PKG_CONFIG=false])
 
 AC_CHECK_PROG(found_ranlib, ranlib, yes, no)
 if test x$found_ranlib != xyes