mingw_w64_pthreads: enable 'static' lib output

This make this 'mingw_w64_pthreads' package suitable for
use when cross compiling rust code from linux to windows.

pkgs/os-specific/windows/mingw-w64/pthreads.nix

@@ -4,6 +4,12 @@ stdenv.mkDerivation {
   name = "${mingw_w64.name}-pthreads";
   inherit (mingw_w64) src meta;
 
+  configureFlags = [
+    # Rustc require 'libpthread.a' when targeting 'x86_64-pc-windows-gnu'.
+    # Enabling this makes it work out of the box instead of failing.
+    "--enable-static"
+  ];
+
   preConfigure = ''
     cd mingw-w64-libraries/winpthreads
   '';