GNU Make: Make `nix-log2xml' support optional.

One question remains: how to make the option available to the user.
`getConfig' is nice but useless as long as `useFromStdenv' prevails.

svn path=/nixpkgs/trunk/; revision=12311

pkgs/development/tools/build-managers/gnumake/default.nix

@@ -1,4 +1,4 @@
-{stdenv, fetchurl}:
+{stdenv, fetchurl, log2xmlSupport ? true}:
 
 stdenv.mkDerivation {
   name = "gnumake-3.81";
@@ -7,11 +7,14 @@ stdenv.mkDerivation {
     md5 = "354853e0b2da90c527e35aabb8d6f1e6";
   };
   
-  patches = [
+  patches =
+    (if log2xmlSupport
         # Provide nested log output for subsequent pretty-printing by
         # nix-log2xml.
-    ./log.patch
-
+        then [ ./log.patch ]
+	else [])
+    ++
+    [
       # Purity: don't look for library dependencies (of the form
       # `-lfoo') in /lib and /usr/lib.  It's a stupid feature anyway.
       # Likewise, when searching for included Makefiles, don't look in