python*Packages.pkgconfig: try avoiding multiple breakages

I don't feel like doing many commits like the parent one,
but feel free to improve...

pkgs/development/python-modules/pkgconfig/default.nix

@@ -11,7 +11,10 @@ buildPythonPackage rec {
 
   checkInputs = [ nose ];
 
-  nativeBuildInputs = [ pkgconfig ];
+  # Apparently many our expressions get pythonPackages.pkgconfig and expect
+  # to have the pkg-config executable on $PATH
+  # For some reason, propagatedNativeBuildInputs don't seem enough.
+  propagatedBuildInputs = [ pkgconfig ];
 
   checkPhase = ''
     nosetests