How to write an implicit rule to cover all sources in a sub directory with more sub directories and build objects in the current directory? Here is an example:

rootdir
├─ src
│  ├─ main.c
│  └─ sub
│     ├─ file.c
│     └─ file.h
└─ Makefile

Let’s say, we have 2 source files src/main.c:

#include <stdio.h>
#include "sub/file.h"

int main(int argc, char *argv[]) {
    printf("2 + 3 = %dn", myadd(2, 3));
    return 0;
}

and src/sub/file.c:

int myadd(int a, int b) {
    return a + b;
}

a Header src/sub/file.h:

int myadd(int a, int b);

And a Makefile:

all: tool

# GENERATED PART START
main.o: src/main.c src/sub/file.h
sub/file.o: src/sub/file.c src/sub/file.h
# GENERATED PART END

%.o: src/%.c
    mkdir -p $(dir $@)
    gcc -c -o $@ $<

tool: main.o sub/file.o
    gcc -I src -o $@ $^

@PHONY: all

My expectation is, that the rules create main.o and sub/file.o and build tool:

rootdir
├─ src
│  ├─ main.c
│  └─ sub
│     ├─ file.c
│     └─ file.h
├─ sub
│  └─ file.o
├─ main.o
├─ Makefile
└─ tool

But make creates only main.o and complains, that the linker cannot find sub/file.o.
What’s wrong here?

Note: If I put the objects in a sub directory obj, and write the implicit rule like obj/%.o: src/%.c, it works fine. But that is not what I need. I have to build the objects in my rootdir.