Merge pull request #936 from andrewharp/patch-1

Build protoc for host platform to enable cross-compilation.

protobuf.bzl

@@ -81,6 +81,7 @@ _proto_gen = rule(
         "deps": attr.label_list(providers = ["proto"]),
         "includes": attr.string_list(),
         "protoc": attr.label(
+            cfg = HOST_CFG,
             executable = True,
             single_file = True,
             mandatory = True,