create importable function for defining zlib

WORKSPACE

 workspace(name = "com_google_protobuf")
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+load("//:protobuf_deps.bzl", "protobuf_deps")
+
+# Load common dependencies.
+protobuf_deps()
 
 new_local_repository(
     name = "submodule_gmock",
@@ -22,14 +26,6 @@ http_archive(
     urls = ["https://github.com/bazelbuild/bazel-skylib/archive/2169ae1c374aab4a09aa90e65efe1a3aad4e279b.tar.gz"],
 )
 
-http_archive(
-    name = "net_zlib",
-    build_file = "//:third_party/zlib.BUILD",
-    sha256 = "c3e5e9fdd5004dcb542feda5ee4f0ff0744628baf8ed2dd5d66f8ca1197cb1a1",
-    strip_prefix = "zlib-1.2.11",
-    urls = ["https://zlib.net/zlib-1.2.11.tar.gz"],
-)
-
 bind(
     name = "python_headers",
     actual = "//util/python:python_headers",
@@ -69,8 +65,3 @@ bind(
     name = "gson",
     actual = "@gson_maven//jar",
 )
-
-bind(
-    name = "zlib",
-    actual = "@net_zlib//:zlib",
-)

examples/WORKSPACE

@@ -41,18 +41,10 @@ http_archive(
     urls = ["https://github.com/bazelbuild/bazel-skylib/archive/2169ae1c374aab4a09aa90e65efe1a3aad4e279b.tar.gz"],
 )
 
-http_archive(
-    name = "net_zlib",
-    build_file = "//:third_party/zlib.BUILD",
-    sha256 = "c3e5e9fdd5004dcb542feda5ee4f0ff0744628baf8ed2dd5d66f8ca1197cb1a1",
-    strip_prefix = "zlib-1.2.11",
-    urls = ["https://zlib.net/zlib-1.2.11.tar.gz"],
-)
+load("@com_google_protobuf//:protobuf_deps.bzl", "protobuf_deps")
 
-bind(
-    name = "zlib",
-    actual = "@net_zlib//:zlib",
-)
+protobuf_deps()
 
 load("@bazel_skylib//lib:versions.bzl", "versions")
+
 versions.check(minimum_bazel_version = "0.5.4")

protobuf_deps.bzl

+"""Load dependencies needed to compile the protobuf library as a 3rd-party consumer."""
+
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+def protobuf_deps():
+    """Loads common dependencies needed to compile the protobuf library."""
+
+    native.bind(
+        name = "zlib",
+        actual = "@net_zlib//:zlib",
+    )
+
+    if "net_zlib" not in native.existing_rules():
+        http_archive(
+            name = "net_zlib",
+            build_file = "@com_google_protobuf//:third_party/zlib.BUILD",
+            sha256 = "c3e5e9fdd5004dcb542feda5ee4f0ff0744628baf8ed2dd5d66f8ca1197cb1a1",
+            strip_prefix = "zlib-1.2.11",
+            urls = ["https://zlib.net/zlib-1.2.11.tar.gz"],
+        )