rename cpp file extension

Makefile

+REPO_DIR=$(shell pwd)
+INCLUDE_DIR=$(REPO_DIR)/include
+SRC_DIR=$(REPO_DIR)/src
+SUBMOD_DIR=$(REPO_DIR)/submodule
+BUILD_DIR=$(REPO_DIR)/build
+
+
 CXX=g++
 CXX_FLAGS=-g -std=c++14 \
           -I/usr/local/include \
-          -I./include \
-          -I./submodule/json/single_include
+          -I$(INCLUDE_DIR)/include \
+          -I$(SUBMOD_DIR)/json/single_include
 LD_FLAGS=-L/usr/local/lib \
          -lexchange_server
-BUILD_DIR=$(shell pwd)/build
 
-XSTEST_SRCS=src/xs-test.cpp
-XSTEST_OBJS=$(addprefix $(BUILD_DIR)/,$(XSTEST_SRCS:.cpp=.o))
+XSTEST_HDRS=$(INCLUDE_DIR)/Config.h
+XSTEST_SRCS=$(SRC_DIR)/xs-test.cc
+XSTEST_OBJS=$(subst $(REPO_DIR),$(BUILD_DIR),$(XSTEST_SRCS:.cc=.o))
 
 all: $(BUILD_DIR)/xs-test
 
-$(BUILD_DIR)/xs-test: $(XSTEST_OBJS)
+$(BUILD_DIR)/xs-test: $(XSTEST_OBJS) $(XSTEST_HDRS)
 	$(CXX) -o $@ $(LD_FLAGS) $(XSTEST_OBJS)
 
-$(BUILD_DIR)/%.o: %.cpp 
+$(BUILD_DIR)/%.o: $(REPO_DIR)/%.cc
 	mkdir -p $(dir $@)
 	$(CXX) -o $@ -c $(CXX_FLAGS) $<