avahi: Add pkgconf as a tool_requires

[jwillikers]

Avahi uses pkg-config to install, so it is required when building. Unfortunately, Avahi doesn't accept pkgconf in place of pkg-config. Overriding Avahi's check solves this problem.

Specify library name and version: avahi/*

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• I've read the contributing guidelines.
• I've used a recent Conan client version close to the currently deployed.
• I've tried at least one configuration locally with the conan-center hook activated.

[conan-center-bot]

Conan v1 pipeline

All green in build 1 (7906977797055735b641e32576914a18a2346677):

• avahi/0.8@:
  All packages built successfully! (All logs)