replace pkg-config with pkgconf

[boegel]

pkg-config has been stuck at 0.29.2 since a long time, but apparently there's a compatible actively maintained replacement: pkgconf, we should look into migrating to it...

[boegel]

See also https://www.mail-archive.com/gentoo-dev@lists.gentoo.org/msg93072.html and https://fedoraproject.org/wiki/Changes/pkgconf_as_system_pkg-config_implementation

[Micket]

We have done this.