capnproto: fix on clang 4

Embedding the patch because the way they build their source releases
changes the paths, so fetchpatch against GitHub doesn't apply cleanly

pkgs/development/libraries/capnproto/clang4.patch

@@ -0,0 +1,22 @@
+From 0f1fd1938b19dccdb5dbfe9cb5177c4342a2a5b5 Mon Sep 17 00:00:00 2001
+From: Eric Fiselier <eric@efcs.ca>
+Date: Thu, 29 Dec 2016 14:23:40 -0700
+Subject: [PATCH] Fix conversion build error when building with Clang 4.0
+
+---
+ c++/src/capnp/layout.h | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/c++/src/capnp/layout.h b/c++/src/capnp/layout.h
+index 850925a..b955f08 100644
+--- a/src/capnp/layout.h
++++ b/src/capnp/layout.h
+@@ -126,7 +126,7 @@template <> struct ElementSizeForType<Void> { static constexpr ElementSize value
+ template <> struct ElementSizeForType<bool> { static constexpr ElementSize value = ElementSize::BIT; };
+ 
+ // Lists and blobs are pointers, not structs.
+-template <typename T, bool b> struct ElementSizeForType<List<T, b>> {
++template <typename T, Kind K> struct ElementSizeForType<List<T, K>> {
+   static constexpr ElementSize value = ElementSize::POINTER;
+ };
+ template <> struct ElementSizeForType<Text> {

pkgs/development/libraries/capnproto/default.nix

@@ -9,6 +9,11 @@ stdenv.mkDerivation rec {
     sha256 = "1yvaadhgakskqq5wpv53hd6fc3pp17mrdldw4i5cvgck4iwprcfd";
   };
 
+  patches = [
+    # Remove once they release a version above 0.5.3. See https://github.com/sandstorm-io/capnproto/issues/433
+    ./clang4.patch
+  ];
+
   meta = with stdenv.lib; {
     homepage    = "http://kentonv.github.io/capnproto";
     description = "Cap'n Proto cerealization protocol";