Add patches option to buildExtension of gawkextlib

With this, it's possible to add patches to the `gawkextlib` source
code in the declaration of the specific extensions that need them.

pkgs/tools/text/gawk/gawkextlib.nix

@@ -5,7 +5,7 @@
 
 let
   buildExtension = lib.makeOverridable
-    ({ name, gawkextlib, extraBuildInputs ? [ ], doCheck ? true }:
+    ({ name, gawkextlib, extraBuildInputs ? [ ], doCheck ? true, patches ? [ ] }:
       let is_extension = gawkextlib != null;
       in stdenv.mkDerivation rec {
         pname = "gawkextlib-${name}";
@@ -17,6 +17,8 @@ let
           sha256 = "sha256-0p3CrQ3TBl7UcveZytK/9rkAzn69RRM2GwY2eCeqlkg=";
         };
 
+        inherit patches;
+
         postPatch = ''
           cd ${name}
         '';