How to properly use -MD flag in Makefile - makefile

Question

I'm trying to create a Makefile where make only recompiles necessary .cpp files.
By default, it works well if I edit .cpp, however, when I edit .h files, it just ignores the changes.
So I've read of -MD flag and its friend -MP (used to avoid bugs when typing make).
However, I can't seem to make it working, if I use -MMD, it works perfectly but I rely on system includes too as I'm writing a library too that is evolving along with the project. Thus, if I update the libary header and reinstall the library, typing make in the main project should recompiles files that includes the changed header.
When using -MMD flag, it - as expected - does not recompile the project, however, using -MD flag, it does recompile everything. In fact, -MD recompiles everything every time, even when nothing changed.
Here is a minimal project structure that reproduce the issue:
./Makefile:
all: build
re: clean build
build: build_lib install_lib build_client
build_lib:
$(MAKE) -C lib
$(MAKE) install -C lib
build_client:
$(MAKE) -C client
install_lib:
$(MAKE) install -C lib
.PHONY: clean
clean: clean_lib clean_client
clean_lib:
$(MAKE) clean -C lib
clean_client:
$(MAKE) clean -C client
./client/Makefile:
CC = g++
INC = -I../lib
CXXFLAGS = -Wall $(INC) -g -MD -MP
EXEC_NAME = ../test
src = $(shell find $(SOURCEDIR) -name '*.cpp')
obj = $(src:.cpp=.o)
LIBRARIES = -ltest_lib
LDFLAGS = -rdynamic $(LIBRARIES)
all: $(EXEC_NAME)
re: clean $(EXEC_NAME)
$(EXEC_NAME): $(obj)
$(CC) -o $# $^ $(LDFLAGS)
-include $(obj:.o=.d)
.PHONY: clean
clean:
rm -f $(obj) $(EXEC_NAME)
./lib/Makefile:
.PHONY : clean
CXXFLAGS= -fPIC -g -Itest_lib/include -MMD -MP
LDFLAGS= -shared
SOURCES = $(shell find $(SOURCEDIR) -name '*.cpp')
HEADERS = $(shell find $(SOURCEDIR) -name '*.h')
OBJECTS=$(SOURCES:.cpp=.o)
TARGET=libtest_lib.so
INC_FOLDER=test_lib/include
CUR_DIR = $(shell pwd)
all: $(TARGET)
install:
sudo rm -rf /usr/local/lib/libtest_lib.so && sudo ln -s $(CUR_DIR)/$(TARGET) /usr/local/lib/libtest_lib.so
sudo rm -rf /usr/local/include/test_lib && sudo cp -r $(INC_FOLDER) /usr/local/include/test_lib
clean:
rm -f $(OBJECTS) $(TARGET)
$(TARGET) : $(OBJECTS)
$(CC) $(CFLAGS) $(OBJECTS) -o $# $(LDFLAGS)
-include $(OBJECTS:.o=.d)
./client/main.cpp:
#include "bar.h"
int main()
{
Bar b;
b.sayHello();
b.sayBye();
return 0;
}
./client/bar.h
#ifndef __BAR__
#define __BAR__
#include <test_lib/foo.h>
#include <iostream>
struct Bar : public Foo
{
Bar() {};
~Bar() {};
void sayBye() const {
std::cout << "Bye " << name << "..." << std::endl;
};
};
#endif
./lib/test_lib/include/foo.h
#ifndef __FOO__
#define __FOO__
struct Foo
{
const char *name;
Foo(const char *name = "world");
~Foo();
void sayHello() const;
};
#endif
./lib/test_lib/src
#include "foo.h"
#include <iostream>
Foo::Foo(const char *name) : name(name) {}
Foo::~Foo() {}
void Foo::sayHello() const
{
std::cout << "Hello " << name << " !" << std::endl;
}

Answer 1

Problem was that I copied the library headers all the time in /usr/local so the files got newer and the client make then thought every header of the library had changed.
A simple fix to this problem was to replace in the library Makefile the following line:
sudo rm -rf /usr/local/include/test_lib && sudo cp -r $(INC_FOLDER) /usr/local/include/test_lib
by
sudo rm -rf /usr/local/include/test_lib && sudo ln -s $(CUR_DIR)/$(INC_FOLDER) /usr/local/include/test_lib
On a side note, provided example is missing the removal of .d files on the clean rule.