tools/pkg-config: Replace with pkgconf

pkgconf is a newer, actively maintained implementation of pkg-config that
supports more aspects of the pkg-config file specification and provides a
library interface that applications can use to incorporate intelligent
handling of pkg-config files into themselves (such as build file
generators, IDEs, and compilers). Through its pkg-config compatibility
interface (activated when it is run as "pkg-config"), it also can
completely replace the original implementation.

It is also lighterweight and does not require glib2, as pkg-config does.

On other distros, pkgconf is symlinked to pkg-config. For simplicity here,
it is renamed to pkg-config.real, as in the original package.

Initial results have been positive. As before, pkgconf works as long as
the pkg-config files point to the proper paths.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[backported upstream fix for Meson]
Signed-off-by: Petr Štetiar <ynezz@true.cz>