title: Modify CMAKE_INSTALL_PREFIX for subdirs
body: <p>I have a project which should be installed at <code>${CMAKE_INSTALL_PREFIX}</code> (supplied by user).  </p>
  
  <p>I include a sub-project with <code>add_subdirectory(subproject)</code>.  This subproject installs content to <code>${CMAKE_INSTALL_PREFIX}</code>, but I'd rather install it to <code>${CMAKE_INSTALL_PREFIX}/subproject</code>.  How can I do that without touching the subproject?</p>
  
  <p>I've tried this: But it had no effect:</p>
  
  <pre><code>set( _INSTALL_PREFIX_BACKUP ${CMAKE_INSTALL_PREFIX})
  set( CMAKE_INSTALL_PREFIX ${CMAKE_INSTALL_PREFIX}/subproject)
  add_subdirectory(subproject)
  set( CMAKE_INSTALL_PREFIX ${_INSTALL_PREFIX_BACKUP})
  </code></pre>
  
  <p>I've also tried this: </p>
  
  <pre><code>include(ExternalProject)
  ExternalProject_Add( subproject
      SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/subproject
      CMAKE_ARGS -DCMAKE_INSTALL_PREFIX="${CMAKE_INSTALL_PREFIX}/subproject"
      INSTALL_DIR "${CMAKE_INSTALL_PREFIX}/subproject"
  )
  </code></pre>