问cmake:如何在只有target_link_directories (没有target_link_libraries)的共享库中设置rpath？

EN

.
├── CMakeLists.txt
├── build # this directory exists to perform an "out-of-source" build with "cmake .."
├── libs
│   ├── CMakeLists.txt
│   ├── bar.c
│   └── foo.c
└── main.c

$ objdump -x ./main | grep PATH
  RUNPATH              /home/user/code/scratch/3/build/libs

$ objdump -x ./libs/libfoo.so | grep PATH
$

# Linking libbar works, but I'd prefer not to do this:
target_link_libraries(foo bar)

$ objdump -x ./libs/libfoo.so | grep PATH
  RUNPATH              /home/user/code/scratch/3/build/libs

# Linking libbar works, but I'd prefer not to do this:
# target_link_libraries(foo bar)

# Linking an unnecessary library, then doing target_link_directories also works:
target_link_libraries(foo dl)
target_link_directories(foo PRIVATE ${CMAKE_CURRENT_BINARY_DIR})

$ objdump -x ./libs/libfoo.so | grep PATH
  RUNPATH              /home/user/code/scratch/3/build/libs

// main.c
#include <dlfcn.h>
#include <stdlib.h>
#include <stdio.h>

int main(int argc, char* argv[]) {
  void* handle = dlopen("libfoo.so", RTLD_LAZY);
  if (!handle) {
    printf("dlopen error: %s\n", dlerror());
    return EXIT_FAILURE;
  }
  {
    void (*fptr)() = dlsym(handle, "func");

    if (fptr) { fptr(); }
  }
  dlclose(handle);
  return EXIT_SUCCESS;
}

// libs/foo.c
#include <dlfcn.h>
#include <stdio.h>

void func() {
  void* handle = dlopen("libbar.so", RTLD_LAZY);
  printf("here in libfoo!\n");
  if (!handle) {
    printf("dlopen error: %s\n", dlerror());
    return;
  }
  {
    void (*fptr)() = dlsym(handle, "func");
    if (fptr) { fptr(); }
  }
  dlclose(handle);
}

// libs/bar.c
#include <stdio.h>

void func() {
  printf("here in libbar!\n");
}

# CMakeLists.txt
cmake_minimum_required(VERSION 3.16)
project(my_prj)

add_subdirectory(libs)

add_executable(main main.c)
target_link_libraries(main dl)
target_link_directories(main PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/libs)

# libs/CMakeLists.txt
add_library(bar SHARED bar.c)

add_library(foo SHARED foo.c)

# This is what I want, but it doesn't work:
target_link_directories(foo PRIVATE ${CMAKE_CURRENT_BINARY_DIR})

# Linking libbar works, but I'd prefer not to do this:
# target_link_libraries(foo bar)

# Linking an unnecessary library, then doing target_link_directories also works:
# target_link_libraries(foo dl)
# target_link_directories(foo PRIVATE ${CMAKE_CURRENT_BINARY_DIR})