Fix gtest build (#13926)

diff --git a/tests/cpp/unittest.mk b/tests/cpp/unittest.mk
index 746ee2f..bca0389 100644
--- a/tests/cpp/unittest.mk
+++ b/tests/cpp/unittest.mk
@@ -25,6 +25,8 @@
 GTEST_HEADERS = $(GTEST_DIR)/include/gtest/*.h \
                 $(GTEST_DIR)/include/gtest/internal/*.h
 
+GTEST_FLAGS=-Wall -Wshadow -DGTEST_HAS_PTHREAD=1 -fexceptions -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -std=c++11
+
 TEST_CFLAGS = -Itests/cpp/include -Isrc $(CFLAGS)
 TEST_LDFLAGS = $(LDFLAGS) -Llib -lmxnet
 
@@ -36,7 +38,7 @@
 .PHONY: runtest testclean
 
 gtest-all.o : $(GTEST_SRCS_)
-	$(CXX) $(CPPFLAGS) -I$(GTEST_INC) -I$(GTEST_DIR) $(CXXFLAGS) -c $(GTEST_DIR)/src/gtest-all.cc
+	$(CXX) $(CPPFLAGS) $(GTEST_FLAGS) -I$(GTEST_INC) -I$(GTEST_DIR) $(CXXFLAGS) -c $(GTEST_DIR)/src/gtest-all.cc
 
 gtest.a : gtest-all.o
 	$(AR) $(ARFLAGS) $@ $^