Add BUILD.gn

Move the build config file into the third_party/googletest repository.
This is useful because it removes a symlink in repo-based builds, but
also because it now builds googletest in its own folder, rather than the
common //third_party folder.

Change-Id: Ic7a533c2287b770caf818ead9ec4b7a73c9fdb42
diff --git a/BUILD.gn b/BUILD.gn
new file mode 100644
index 0000000..70c2577
--- /dev/null
+++ b/BUILD.gn
@@ -0,0 +1,52 @@
+# Copyright 2018 The Hafnium Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+config("gtest_config") {
+  visibility = [ ":gtest" ]
+
+  include_dirs = [
+    "googletest/include",
+    "googlemock/include",
+  ]
+
+  ldflags = [ "-pthread" ]
+}
+
+source_set("gtest") {
+  testonly = true
+
+  sources = [
+    "googlemock/src/gmock-all.cc",
+    "googletest/src/gtest-all.cc",
+  ]
+
+  include_dirs = [
+    "googletest",
+    "googlemock",
+  ]
+
+  all_dependent_configs = [ ":gtest_config" ]
+}
+
+source_set("gtest_main") {
+  testonly = true
+
+  sources = [
+    "googlemock/src/gmock_main.cc",
+  ]
+
+  deps = [
+    ":gtest",
+  ]
+}