Use PKG_CONFIG variables for pkg-config

[Ericson2314]

When cross compiling, one often has a prefixed pkg-config executable that uses a different search path, so host platform and build platform pkg-config files don't get mixed up.

(This is upstreaming a fix made for the distro I use, NixOS, in https://github.com/NixOS/nixpkgs/pull/91109.)