title: cmake : use ExternalProject_Add and specify the output folder
body: <p>I'm currently using one CMakeLists.txt file that will execute an external CMakeLists.txt (and dependency).</p>
  
  <p>For this I use the ExternalProject_Add command, but I can't find a way to specify where the ".sln" file will be generated.</p>
  
  <p>Here is the command I use :</p>
  
  <pre><code>include(ExternalProject)
  set(LIBRARY_SOURCE_DIR C:/Library)
  ExternalProject_Add(Library
    SOURCE_DIR        "${LIBRARY_SOURCE_DIR}"
    CMAKE_ARGS
        -DCMAKE_BUILD_TARGET_ANDROID:BOOL=ON
        -DCMAKE_CXX_FLAGS=${CMAKE_CXX_FLAGS} 
        -DCMAKE_C_FLAGS=${CMAKE_C_FLAGS} 
        -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
        -DCMAKE_CONFIGURATION_TYPES=${CMAKE_CONFIGURATION_TYPES}
        -DCMAKE_TOOLCHAIN_FILE=${CMAKE_TOOLCHAIN_FILE}
        -G@CMAKE_GENERATOR@
    BUILD_COMMAND     ""
  )
  </code></pre>
  
  <p>So, I looking for a way to directly specify it in this call. Any idea ?</p>
  
  <p>I also tried, but no way:</p>
  
  <pre><code>  PREFIX            LibraryBuild
    SOURCE_DIR        "${LIBRARY_SOURCE_DIR}"
    BINARY_DIR        "${LIBRARY_SOURCE_DIR}/LibraryBuild"
    INSTALL_DIR       "${LIBRARY_SOURCE_DIR}/LibraryBuild"
    BUILD_COMMAND     "cmake --build ./LibraryBuild"
    CMAKE_ARGS
        -DCMAKE_RUNTIME_OUTPUT_DIRECTORY="./LibraryBuild"
        -DCMAKE_LIBRARY_OUTPUT_DIRECTORY="./LibraryBuild"
        -DCMAKE_EXECUTABLE_OUTPUT_DIRECTORY="./LibraryBuild"
        "./LibraryBuild"
        -DCMAKE_CURRENT_BINARY_DIR="./LibraryBuild"
        -DCMAKE_GENERATOR="Visual Studio 15 2017 Win64"
  </code></pre>