gpg/pth: fixing the pth building on arm

I could built pth with the fix commented in:
http://forums.gentoo.org/viewtopic-t-904280-start-0.html

And I remove the 'pth.supported' structure I had built.

pkgs/development/libraries/pth/default.nix

@@ -8,9 +8,9 @@ stdenv.mkDerivation rec {
     sha256 = "0ckjqw5kz5m30srqi87idj7xhpw6bpki43mj07bazjm2qmh3cdbj";
   };
 
-  passthru = {
-    supported = ! stdenv.isArm;
-  };
+  preConfigure = stdenv.lib.optionalString stdenv.isArm ''
+    configureFlagsArray=("CFLAGS=-DJB_SP=8 -DJB_PC=9")
+  '';
 
   meta = {
     description = "The GNU Portable Threads library";