我是CMake的新手，我想使用cmake构建一个项目（project_a），其中我必须包含一个外部autotools项目（project_b），而我必须将它与GSL库链接。这是我的CMakeLists.txt的一部分：
cmake_minimum_required(VERSION 2.6 FATAL_ERROR)
project(project_a)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-shadow")
ExternalProject_Add(project_b
SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/project_b
CONFIGURE_COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/project_b/configure --prefix=<INSTALL_DIR>
BUILD_COMMAND ${MAKE})
find_package(GSL REQUIRED)
include_directories(${GSL_INCLUDE_DIRS})
set(PROJECT_B_LIB_DIR "${PROJECT_SOURCE_DIR}/build/project_b-prefix/lib")
set(prefix "lib")
set(suffix ".a")
set(PROJECT_B_LIBRARIES "${PROJECT_B_LIB_DIR}/${prefix}project_b${suffix}")
target_link_libraries(project_a ${GSL_LIBRARIES})
target_link_libraries(project_a ${PROJECT_B_LIBRARIES})
问题是GSL库在project_a中工作，但是在project_b中我得到了我尝试使用的任何GSL函数的“未定义引用”。