boost: build Python numpy extension by default

In order to manipulate Python arrays numpy is needed from boost 1.65 on.
http://www.boost.org/users/history/version_1_65_1.html

pkgs/development/libraries/boost/1.65.nix

@@ -9,4 +9,6 @@ callPackage ./generic.nix (args // rec {
     sha256 = "9807a5d16566c57fd74fb522764e0b134a8bbe6b6e8967b83afefd30dcd3be81";
   };
 
+  enableNumpy = true;
+
 })

pkgs/development/libraries/boost/generic.nix

@@ -10,7 +10,7 @@
 , enablePIC ? false
 , enableExceptions ? false
 , enablePython ? hostPlatform == buildPlatform
-, enableNumpy ? false, numpy ? null
+, enableNumpy ? false
 , taggedLayout ? ((enableRelease && enableDebug) || (enableSingleThreaded && enableMultiThreaded) || (enableShared && enableStatic))
 , patches ? null
 , mpi ? null
@@ -156,7 +156,7 @@ stdenv.mkDerivation {
     ++ optional (hostPlatform == buildPlatform) icu
     ++ optional stdenv.isDarwin fixDarwinDylibNames
     ++ optional enablePython python
-    ++ optional enableNumpy numpy;
+    ++ optional enableNumpy python.pkgs.numpy;
 
   configureScript = "./bootstrap.sh";
   configureFlags = commonConfigureFlags