diff --git a/cpp/helloworld/Makefile b/cpp/helloworld/Makefile
index 83f2ad4..5a7c34f 100644
--- a/cpp/helloworld/Makefile
+++ b/cpp/helloworld/Makefile
@@ -30,9 +30,9 @@
 #
 
 CXX = g++
-CPPFLAGS = -I/usr/local/include -pthread
-CXXFLAGS = -std=c++11
-LDFLAGS = -L/usr/local/lib -lgrpc -lgrpc++ -lprotobuf -lpthread -ldl
+CPPFLAGS += -I/usr/local/include -pthread
+CXXFLAGS += -std=c++11
+LDFLAGS += -L/usr/local/lib -lgrpc -lgrpc++ -lprotobuf -lpthread -ldl
 PROTOC = protoc
 GRPC_CPP_PLUGIN = grpc_cpp_plugin
 GRPC_CPP_PLUGIN_PATH ?= `which $(GRPC_CPP_PLUGIN)`
diff --git a/cpp/route_guide/Makefile b/cpp/route_guide/Makefile
index 1319562..ea02944 100644
--- a/cpp/route_guide/Makefile
+++ b/cpp/route_guide/Makefile
@@ -30,9 +30,9 @@
 #
 
 CXX = g++
-CPPFLAGS = -I/usr/local/include -pthread
-CXXFLAGS = -std=c++11
-LDFLAGS = -L/usr/local/lib -lgpr -lgrpc -lgrpc++ -lprotobuf -lpthread -ldl
+CPPFLAGS += -I/usr/local/include -pthread
+CXXFLAGS += -std=c++11
+LDFLAGS += -L/usr/local/lib -lgpr -lgrpc -lgrpc++ -lprotobuf -lpthread -ldl
 PROTOC = protoc
 GRPC_CPP_PLUGIN = grpc_cpp_plugin
 GRPC_CPP_PLUGIN_PATH ?= `which $(GRPC_CPP_PLUGIN)`