Fix build from git

As issue #55 (closed) already showed, it's possible not possible to build pkg-config from git, because the ./autogen.sh/autoreconf fails due to obsolete glib-m4 macros in the internal glib copy. This patch fixes building from git by dropping the internal copy of glib.

While fixing/updating the internal copy of glib to build again would have been ideal, I'm afraid this is not realistically feasible (as far as I can tell; if you can come up with something better that would of course be very welcome). The major issue being that glib removed its autotools buildsystem replacing it with a Meson one (and Meson depends on Python whose build uses pkg-config). Unless one were to fully rewrite a autotools or manual POSIX-only buildsystem and continue to maintain it for recent glib bootstrapping won't get any easier with a Meson-dependent internal glib. (At this point it would probably easier to instead rewrite pkg-config to no longer rely on glib.)

Thus, rather than being unable to build pkg-config at all, we drop the internal glib. 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

*(Needed to do this to be able to build and test my Cflags.private merge request !13 *