pari: Make pthread optional, remove unstable

pkgs/applications/science/math/pari/default.nix

@@ -1,5 +1,9 @@
 { stdenv, fetchurl
-, gmp, readline, libX11, libpthreadstubs, tex, perl }:
+, gmp, readline, libX11, tex, perl
+, withThread ? true, libpthreadstubs
+}:
+
+assert withThread -> libpthreadstubs != null;
 
 stdenv.mkDerivation rec {
 
@@ -11,14 +15,22 @@ stdenv.mkDerivation rec {
     sha256 = "0ir6m3a8r46md5x6zk4xf159qra7aqparby9zk03k81hjrrxr72g";
   };
 
-  buildInputs = [ gmp readline libX11 libpthreadstubs tex perl ];
+  buildInputs = [
+    gmp
+    readline
+    libX11
+    tex
+    perl
+  ] ++ stdenv.lib.optionals withThread [
+    libpthreadstubs
+  ];
 
   configureScript = "./Configure";
   configureFlags = [
-    "--mt=pthread"
     "--with-gmp=${gmp.dev}"
     "--with-readline=${readline.dev}"
-  ] ++ stdenv.lib.optional stdenv.isDarwin "--host=x86_64-darwin";
+  ] ++ stdenv.lib.optional stdenv.isDarwin "--host=x86_64-darwin"
+  ++ stdenv.lib.optional withThread "--mt=pthread";
 
   preConfigure = ''
     export LD=$CC