graphviz: patch vimdot to lookup 'vim' in $PATH

Without this it tries to run /usr/bin/vim. Patch is already accepted
upstream.

pkgs/tools/graphics/graphviz/0001-vimdot-lookup-vim-in-PATH.patch

@@ -0,0 +1,30 @@
+From 2008bf62e13ebe41cdad3e16f8b42f46ae393876 Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Bj=C3=B8rn=20Forsman?= <bjorn.forsman@gmail.com>
+Date: Tue, 6 Jan 2015 20:39:41 +0100
+Subject: [PATCH] vimdot: lookup 'vim' in $PATH
+
+Instead of hardcoding /usr/bin/vim. Needed for NixOS (http://nixos.org), and is
+probably useful for others too.
+---
+ plugin/xlib/vimdot.sh | 4 ++--
+ 1 file changed, 2 insertions(+), 2 deletions(-)
+
+diff --git a/plugin/xlib/vimdot.sh b/plugin/xlib/vimdot.sh
+index 749fe6a..4e6dd4b 100755
+--- a/plugin/xlib/vimdot.sh
++++ b/plugin/xlib/vimdot.sh
+@@ -3,9 +3,9 @@
+ 
+ error() { echo "$0: $*" >&2; exit 1; }
+ 
+-editor="/usr/bin/vim"
++editor="vim"
+ 
+-if ! test -x "$editor"; then error "the \"$editor\" editor not found or not executable"; fi
++if ! test -x "$(command -v "$editor")"; then error "the \"$editor\" editor not found or not executable"; fi
+ 
+ default="noname.gv"
+ 
+-- 
+2.1.3
+

pkgs/tools/graphics/graphviz/default.nix

@@ -11,6 +11,8 @@ stdenv.mkDerivation rec {
     sha256 = "17l5czpvv5ilmg17frg0w4qwf89jzh2aglm9fgx0l0aakn6j7al1";
   };
 
+  patches = [ ./0001-vimdot-lookup-vim-in-PATH.patch ];
+
   buildInputs =
     [ pkgconfig libpng libjpeg expat libXaw yacc libtool fontconfig
       pango gd gts