third_party: Add cython system dep

Signed-off-by: Jason Zaman <jason@perfinion.com>

tensorflow/workspace.bzl

@@ -707,6 +707,7 @@ def tf_workspace(path_prefix="", tf_repo_name=""):
       strip_prefix = "cython-0.28.4",
       build_file = clean_dep("//third_party:cython.BUILD"),
       delete = ["BUILD.bazel"],
+      system_build_file = clean_dep("//third_party/systemlibs:cython.BUILD"),
   )
 
   tf_http_archive(

third_party/systemlibs/cython.BUILD

@@ -0,0 +1,13 @@
+licenses(["notice"])  # Apache-2.0
+
+genrule(
+    name = "lncython",
+    outs = ["cython"],
+    cmd = "ln -s $$(which cython) $@",
+)
+
+sh_binary(
+    name = "cython_binary",
+    srcs = ["cython"],
+    visibility = ["//visibility:public"],
+)