common.mk: sync w/platform2

Bug: chromium:769107
Change-Id: I43ee1ed925c10da2c23e09859131c453a32265a8
diff --git a/Makefile b/Makefile
index 43790a5..b0f0509 100644
--- a/Makefile
+++ b/Makefile
@@ -37,7 +37,7 @@
 
 USE_SYSTEM_GTEST ?= no
 ifeq ($(USE_SYSTEM_GTEST),no)
-GTEST_CXXFLAGS := -std=gnu++11
+GTEST_CXXFLAGS := -std=gnu++14
 GTEST_LIBS := gtest.a
 else
 GTEST_CXXFLAGS := $(shell gtest-config --cxxflags)