postgresql.pg_config: add check to confirm output is correct

This is only tested on native builds against the output of the original
pg_config, which should be enough.

pkgs/servers/sql/postgresql/generic.nix

···
 
       postPatch = ''
         substituteInPlace "src/Makefile.global.in" --subst-var out
+        substituteInPlace "src/common/config_info.c" --subst-var dev
         cat ${./pg_config.env.mk} >> src/common/Makefile
       '';
 
···
         ''
           moveToOutput "bin/ecpg" "$dev"
           moveToOutput "lib/pgxs" "$dev"
-
+        ''
+        + lib.optionalString (stdenv'.buildPlatform.canExecute stdenv'.hostPlatform) ''
+          mkdir -p "$dev/nix-support"
+          "$out/bin/pg_config" > "$dev/nix-support/pg_config.expected"
+        ''
+        + ''
           rm "$out/bin/pg_config"
           make -C src/common pg_config.env
           install -D src/common/pg_config.env "$dev/nix-support/pg_config.env"

pkgs/servers/sql/postgresql/pg_config.nix

···
 {
+  diffutils,
   lib,
   replaceVarsWith,
   runtimeShell,
+  stdenv,
   # PostgreSQL package
   finalPackage,
 }:
···
     inherit runtimeShell;
     postgresql-dev = lib.getDev finalPackage;
   };
+  nativeCheckInputs = [
+    diffutils
+  ];
+  postCheck = ''
+    if [ -e ${lib.getDev finalPackage}/nix-support/pg_config.expected ]; then
+        diff ${lib.getDev finalPackage}/nix-support/pg_config.expected <($out/bin/pg_config)
+    fi
+  '';
 }