third_party: unbundle nsync

This adds to PR 20284 and enabled nsync to use the system libs.
The required changes are in nsync-1.20.1.

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

tensorflow/workspace.bzl

@@ -400,6 +400,7 @@ def tf_workspace(path_prefix = "", tf_repo_name = ""):
         ],
         sha256 = "692f9b30e219f71a6371b98edd39cef3cbda35ac3abc4cd99ce19db430a5591a",
         strip_prefix = "nsync-1.20.1",
+        system_build_file = clean_dep("//third_party/systemlibs:nsync.BUILD"),
     )
 
     tf_http_archive(

third_party/systemlibs/nsync.BUILD

@@ -0,0 +1,23 @@
+licenses(["notice"])  # BSD 3-Clause
+
+filegroup(
+    name = "LICENSE",
+    visibility = ["//visibility:public"],
+)
+
+cc_library(
+    name = "nsync_headers",
+    visibility = ["//visibility:public"],
+)
+
+cc_library(
+    name = "nsync",
+    linkopts = ["-lnsync"],
+    visibility = ["//visibility:public"],
+)
+
+cc_library(
+    name = "nsync_cpp",
+    linkopts = ["-lnsync_cpp"],
+    visibility = ["//visibility:public"],
+)

third_party/systemlibs/syslibs_configure.bzl

@@ -22,6 +22,7 @@ VALID_LIBS=[
     "jsoncpp_git",
     "lmdb",
     "nasm",
+    "nsync",
     "org_sqlite",
     "pcre",
     "png_archive",