我有很多子文件夹

home
|
|-library1
|-library2
|
|-libraryn

每个子文件夹包含一个完整的库，可以自己编译（每个库有一个不同的mantainer）。到目前为止，它的工作正常，我编译他们使用脚本。
现在我需要创建另一个依赖于现有库的库，为此，我在home文件夹下创建了一个CMakeLists.txt，使用add_subdirectory命令可以编译所有库。
我有类似

cmake_minimum_required (VERSION 2.8)

add_subdirectory(library1)
add_subdirectory(library2)
...
add_subdirectory(libraryn)

当我尝试执行cmake时，我获得了以下各种库的错误：

CMake Error at libraryY/CMakeLists.txt:63 (add_custom_target):
  add_custom_target cannot create target "doc" because another target with
  the same name already exists.  The existing target is a custom target
  created in source directory
  "/path/to/libraryX".  See
  documentation for policy CMP0002 for more details.

这是因为在每个库中我们创建了一个doc目标来编译库本身的Doxygen文档，当libraryes一个一个编译时它工作得很好，但是对于master CMakeLists.txt我似乎做不到。

# Create doc target for doxygen documentation compilation.
find_package (Doxygen)
if (DOXYGEN_FOUND)
  set (Doxygen_Dir ${CMAKE_BINARY_DIR}/export/${Library_Version}/doc)
  # Copy images folder
  file (GLOB IMAGES_SRC "images/*")
  file (COPY ${IMAGES_SRC} DESTINATION ${CMAKE_CURRENT_BINARY_DIR}/images)
  configure_file (${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile @ONLY)
  add_custom_target (doc
    ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile
    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
    COMMENT "Generating doxygen documentation" VERBATIM
  )
else (DOXYGEN_FOUND)
  message (STATUS "Doxygen must be installed in order to compile doc")
endif (DOXYGEN_FOUND)

有没有一种方法可以在不修改这个目标的情况下立即编译这些项目？