使用CMAKE添加仅汇编依赖项

我有一个简单的项目，该项目需要三个仅标题库才能编译：websocketpp，spdlog和nlohmann/json。

项目结构看起来像这样：

└── src
    ├── app
    │   ├── CMakeLists.txt
    │   ├── src
    │   └── test
    ├── CMakeLists.txt
    ├── core
    │   ├── CMakeLists.txt
    │   ├── include
    │   ├── src
    │   └── test
    └── vendor
        ├── install.cmake
        ├── nlohmann_json
        ├── spdlog
        └── websocketpp

root cmakelists.txt如下：

cmake_minimum_required(VERSION 3.6.1 FATAL_ERROR)
..
# External 3rd party libs that we include
include(vendor/install.cmake)
add_subdirectory(core)
add_subdirectory(app)

基本上，每个子目录都是一个库（例如 core），而 app"汇总"全部。每个库（例如core）都是这样构建的（core/CMakeLists.txt）：

project(foo-core VERSION 0.1 LANGUAGES CXX)
add_library(foo-core
  src/foobar/foobar.cc
  src/foobaz/baz.cc)
target_include_directories(foo-core PUBLIC
  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
  $<INSTALL_INTERFACE:include>
  PRIVATE src)
target_link_libraries(foo-core websocketpp spdlog) # <- see here, using spdlog & websocketpp
# 'make install' to the correct location
install(TARGETS foo-core EXPORT FooCoreConfig
  ARCHIVE  DESTINATION lib
  LIBRARY  DESTINATION lib
  RUNTIME  DESTINATION bin)
install(DIRECTORY include/ DESTINATION include)
install(EXPORT FooCoreConfig DESTINATION share/FooCore/cmake)
export(TARGETS foo-core FILE FooCoreConfig.cmake)

注意我如何链接依赖项（仅标题库！）。这就是我获取他们的方式（vendor/install.cmake）：

# spdlog
if((NOT SPDLOG_INCLUDE_DIR) OR (NOT EXISTS ${SPDLOG_INCLUDE_DIR}))
  message("Unable to find spdlog, cloning...")
  execute_process(COMMAND git submodule update --init -- vendor/spdlog
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
  set(SPDLOG_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/vendor/spdlog/include/
    CACHE PATH "spdlog include directory")
  install(DIRECTORY ${SPDLOG_INCLUDE_DIR}/spdlog DESTINATION include)
  # Setup a target
  add_library(spdlog INTERFACE)
  target_include_directories(spdlog INTERFACE
    $<BUILD_INTERFACE:${SPDLOG_INCLUDE_DIR}>
    $<INSTALL_INTERFACE:include>)
  install(TARGETS spdlog EXPORT spdlog DESTINATION include)
endif()
# websocketpp
if((NOT WEBSOCKETPP_INCLUDE_DIR) OR (NOT EXISTS ${WEBSOCKETPP_INCLUDE_DIR}))
  message("Unable to find websocketpp, cloning...")
  execute_process(COMMAND git submodule update --init -- vendor/websocketpp
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
  set(WEBSOCKETPP_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/vendor/websocketpp/
    CACHE PATH "websocketpp include directory")
  install(DIRECTORY ${WEBSOCKETPP_INCLUDE_DIR}/websocketpp DESTINATION include)
  # Setup a target
  add_library(websocketpp INTERFACE)
  target_include_directories(websocketpp INTERFACE
    $<BUILD_INTERFACE:${WEBSOCKETPP_INCLUDE_DIR}>
    $<INSTALL_INTERFACE:include>)
  install(TARGETS websocketpp EXPORT websocketpp DESTINATION include)
endif()
# nlohmann/json
if((NOT NLOHMANN_JSON_INCLUDE_DIR) OR (NOT EXISTS ${NLOHMANN_JSON_INCLUDE_DIR}))    
  message("Unable to find nlohmann/json, cloning...")
  execute_process(COMMAND git submodule update --init -- vendor/nlohmann_json
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
  set(NLOHMANN_JSON_INCLUDE_DIR
    ${CMAKE_CURRENT_SOURCE_DIR}/vendor/nlohmann_json/src/
    CACHE PATH "nlohmann/json include directory")
  install(FILES ${NLOHMANN_JSON_INCLUDE_DIR}/json.hpp DESTINATION include)
  # Setup a target
  add_library(nlohmann_json INTERFACE )
  target_include_directories(nlohmann_json INTERFACE
    $<BUILD_INTERFACE:${NLOHMANN_JSON_INCLUDE_DIR}>
    $<INSTALL_INTERFACE:include>)
  install(TARGETS nlohmann_json EXPORT nlohmann_json DESTINATION include)
endif()

到目前为止很好：您可以看到依赖项被以git子模型为单位，值得庆幸的是，这使管理它们更容易。但是，当我用mkdir build && cd build && cmake ../src编译项目时，我会有以下错误：

cmake错误：安装（导出foocoreconfig ...）包括目标 需要不在导出中的目标WebSocketpp的foo-core 设置。

cmake错误：安装（导出foocoreconfig ...）包括目标 foo-core需要不在导出集中的目标spdlog。

包括标题，例如#include <spdlog/spdlog.h>或#include <nlohmann/json.hpp>产生错误，说找不到标题。

说实话，我对Cmake不太满意，并且在过去的两天里，我一直在调试。这可能很简单，但是我不知道如何实现它。实际上，一个人只会通过-i作为编译器标志来使用我想要的图书馆，但是cmake抽象似乎使我感到困惑。如果有人能够解释为什么这不起作用，我会很高兴，希望将这些图书馆纳入我的项目的正确方法是什么。预先感谢！