C++ Makefile on "complex" tree structure - c++

Question

I have found a lot of related questions, but I was still not able to make my own Makefile.
This makefile is using Mingw64 on Windows, and I want it to run on *nix, currently Debian, but I would like to be able to make it run on Alpine too, as it's used in a Docker container.
The project tree structure is something like :
./
src/
main.cpp
Server.cpp <- use asio and Utils/Split.h
Server.h <- use asio
Utils/
Split.h
lib/
asio/include/ <- asio library (without boost, header only)
Makefile <- That is what I am trying to do right now
Dockerfile
I tried multiple things, here is my latest Makefile (that obviously, does not work) :
NAME := GameServer
CXX := g++
CXXFLAGS := -std=c++2a -DASIO_STANDALONE
SRC_DIR := ./src
LIBS := -I lib/asio-1.18.1/include \
-I lib/rapidjson-1.1.0/include \
-I src
rwildcard = $(wildcard $1$2) $(foreach d,$(wildcard $1*),$(call rwildcard,$d/,$2))
SRCS := $(call rwildcard,$(SRC_DIR),*.cpp)
OBJS := $(SRCS:%.cpp=%.o)
.PHONY: all
all: $(NAME)
$(NAME): $(OBJS)
$(CXX) -o $# $^
$(OBJS): $(SRCS)
$(CXX) $(CXXFLAGS) -c -o $# $< $(LIBS)
Note: the code (.cpp, .h) is valid, it's coming from an already-running project, but built on Visual Studio (compiled with MSVC).
Here are the two functions done my mingw32-make :
g++ -c -o src/Server.o src/main.cpp -I lib/asio/include -I src
g++ -o Server src/main.o src/Server.o
First line : It should builds the .o from the .cpp and adds the include to asio. I added -I src to add src/Utils, but I guess that's not the way of doing it ?
Second line : It should (link ?) the two .o in a single file : the executable.
The errors I am getting with this makefile are :
src/Server.o:main.cpp:(.text+0x36): multiple definition of 'main', src/main.o:main.cpp:(.text+0x36): first defined here (and this, for every .o)
src/main.o:main.cpp:(.text+0x4b): undefined reference to 'Server::Server()' (and this, for every Server methods main calls, even some from asio)
They appears when the second g++ line starts (g++ -o Server src/main.o src/Server.o)
So here are my questions :
What am I doing wrong ?
Is there a better way of trying to make a development environment on Windows and still be able to copy the project in a Docker container (and then compile it with the gcc image) to build it with the same Makefile ?
Sorry if I forgot to mention some details, I am new with Mingw and its environment.
Thank you
Edit : Corrected version :
NAME := GameServer
CXX := g++
CXXFLAGS := -std=c++1z
SRC_DIR := ./src
LIBS := -lwsock32 -lws2_32 \
-I lib/asio-1.18.1/include \
-I lib/rapidjson-1.1.0/include \
-I src
rwildcard = $(wildcard $1$2) $(foreach d,$(wildcard $1*),$(call rwildcard,$d/,$2))
SRCS := $(call rwildcard,$(SRC_DIR),*.cpp)
OBJS := $(SRCS:%.cpp=%.o)
.PHONY: all
all: $(NAME)
$(NAME): $(OBJS)
$(CXX) -o $# $^ $(LIBS)
$(OBJS): $(SRC_DIR)/%.o: $(SRC_DIR)/%.cpp
$(CXX) $(CXXFLAGS) -c -o $# $< $(LIBS)

Answer 1

Consider the rule...
$(OBJS): $(SRCS)
$(CXX) $(CXXFLAGS) -c -o $# $< $(LIBS)
This tells make that all items in $(OBJS) depend on all items in $(SRCS). But the command...
$(CXX) $(CXXFLAGS) -c -o $# $< $(LIBS)
...always compiles the first dependency as identified by $<. It just so happens that in your case $< is src/main.cpp.
Instead you should probably use a pattern rule such as...
$(SRC_DIR)/%.o: $(SRC_DIR)/%.cpp
$(CXX) $(CXXFLAGS) -c -o $# $< $(LIBS)
You can also limit the scope of that rule to only those targets specified by $(OBJS) with a full static pattern rule...
$(OBJS): $(SRC_DIR)/%.o: $(SRC_DIR)/%.cpp
$(CXX) $(CXXFLAGS) -c -o $# $< $(LIBS)