mcgrid: pkgconfig is the only way to locate mcgrid

pkgs/development/libraries/physics/mcgrid/default.nix

···
-{ stdenv, fetchurl, fastnlo, rivet, sherpa }:
+{ stdenv, fetchurl, fastnlo, rivet, pkgconfig, sherpa }:
 
 stdenv.mkDerivation rec {
   name = "mcgrid-${version}";
···
   };
 
   buildInputs = [ fastnlo rivet ];
+  propagatedNativeBuildInputs = [ pkgconfig ];
 
   preConfigure = ''
     substituteInPlace mcgrid.pc.in \