[GN] Add BUILD.gn file

Change-Id: Ia2d532ccbe7d656839b8646f1c4af0223df09923
diff --git a/BUILD.gn b/BUILD.gn
new file mode 100644
index 0000000..64ffd77
--- /dev/null
+++ b/BUILD.gn
@@ -0,0 +1,15 @@
+# Copyright 2020 The Fuchsia Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+config("farmhash_config") {
+  defines = [ "NAMESPACE_FOR_HASH_FUNCTIONS=farmhash" ]
+}
+
+source_set("farmhash") {
+  sources = [
+    "src/farmhash.cc",
+    "src/farmhash.h",
+  ]
+  public_configs = [ ":farmhash_config" ]
+}