swift: init at 3.1

Add dependency 'libblocksruntime'.

pkgs/development/compilers/swift/purity.patch

@@ -0,0 +1,16 @@
+--- a/lib/Driver/Tools.cpp	2016-08-25 15:48:05.187553443 +0200
++++ b/lib/Driver/Tools.cpp	2016-08-25 15:48:47.534468882 +0200
+@@ -9420,13 +9420,6 @@
+   if (!Args.hasArg(options::OPT_static)) {
+     if (Args.hasArg(options::OPT_rdynamic))
+       CmdArgs.push_back("-export-dynamic");
+-
+-    if (!Args.hasArg(options::OPT_shared)) {
+-      const std::string Loader =
+-          D.DyldPrefix + ToolChain.getDynamicLinker(Args);
+-      CmdArgs.push_back("-dynamic-linker");
+-      CmdArgs.push_back(Args.MakeArgString(Loader));
+-    }
+   }
+ 
+   CmdArgs.push_back("-o");