Update gtest

Change-Id: Ifce12b71b7435df58f9b25ef5b5e4e8d0ab909c6
Reviewed-on: https://weave-review.googlesource.com/2330
Reviewed-by: Vitaly Buka <vitalybuka@google.com>
diff --git a/third_party/third_party.mk b/third_party/third_party.mk
index f092e89..8a11e2d 100644
--- a/third_party/third_party.mk
+++ b/third_party/third_party.mk
@@ -50,7 +50,10 @@
 ###
 # libgtest and libgmock (third_party, downloaded on build)
 
-third_party/include/gtest/gtest.h :
+third_party/lib/gtest.a: third_party/include/gtest/gtest.h
+third_party/lib/gmock.a: third_party/include/gtest/gtest.h
+
+third_party/include/gtest/gtest.h:
 	@echo Downloading and building libgtest and libgmock...
 	third_party/get_gtest.sh
 	@echo Finished downloading and building libgtest and libgmock.