The macros in the currently bundled version of glib cause errors during
the initial autogen step. Updating the bundled glib fixes this part, but
breaks confgiure and build with --with-internal-glib as glib changed its
buildsystem to Meson.
Adapting the internal copy to build again would be a considerable effort
and unless glib's buildsystem is completlty rewritten from scratch to
only use POSIX utilities again (after the autogen step) there is no
real benefit in doing so, since the build would still depend on Meson
which depends on Python which uses pkg-config during its build.

However, glib's switch to Meson at least broke the shorter
dependency-cycle of pkg-config dependeing on glib whose builds
"depended" on pkg-config, as Meson will fallback to downloading the
sources of, building and managing all dependencies (including transitive
deps) by itself.
Though even before, this "dependency" was easily avoided by specifying
*_LIBS and *_CFLAGS manually during compile, just as for pkg-config's
own build.

Thus, rather than being unable to build pkg-config at all, we drop the
internal glib (which was previously never strictly required for
bootstrapping merely a convenience) fixing pkg..config's build.
To bootstrap pkg-config from scratch in the future, one still has
multiple options:
 - first build an older version of pkg-config with internal glib
   from its release tarball
 - first start off with an alternative implementation which only depends
   on POSIX (possibly pkgconf does that, but I'm not completly sure)
 - manually specify *_LIBS and *_CFLAGS for python and its dependencies
   and then build glib with Meson but without pkg-config

This obsoletes and therefore closes: pkg-config/pkg-config#29