title: cmake one module include another one
body: <p>For a cmake toolchain file, A.toolchain.cmake, I want to include B.cmake, which only has some variables set. Two cmake file lay in the same folder.
  But cmake output error, cannot find B.cmake.</p>
  
  <blockquote>
    <p>CMake Error at /Users/robert/workspace/cpp/CLIon/rSock/xbuild/cmake/Darwin_x86_64.toolchain.cmake:12 (include):
      include could not find load file:
       /Users/robert/workspace/cpp/CLIon/rSock/xbuild/build/Darwin_x86_64/CMakeFiles/CMakeTmp/xbuild/cmake/base.cmake</p>
  </blockquote>
  
  <p>I tried many ways, relative or absolute path, or set as module. They just don't work. But if I put include in CMakelists.txt, it works.
  Why? Only cmakelists.txt can include other cmake files?</p>
  
  <pre><code>#include("./base.cmake")
  
  #set(CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/xbuild/cmake")
  #include(base)
  
  include(${PROJECT_SOURCE_DIR}/xbuild/cmake/base.cmake)
  </code></pre>