php: Add mcrypt optional support

Use an override with mcrypt = true to enable libmcrypt support in php.
The --disable-posix-threadsds mcrypt build option was required by php.

pkgs/development/libraries/libmcrypt/default.nix

@@ -1,4 +1,6 @@
-{ stdenv, fetchurl }:
+{ stdenv, fetchurl, disablePosixThreads ? false }:
+
+with stdenv.lib;
 
 stdenv.mkDerivation rec {
   name = "libmcrypt-2.5.8";
@@ -10,7 +12,12 @@ stdenv.mkDerivation rec {
 
   buildInputs = [];
 
+  configureFlags = optional disablePosixThreads
+    [ "--disable-posix-threads" ];
+
   meta = {
+    description = "MCrypt is a replacement for the old crypt() package and crypt(1) command, with extensions.";
     homepage = http://mcrypt.sourceforge.net;
+    license = "GPL";
   };
 }