title: Set WORKING_DIRECTORY for ExternalProject_Add CONFIGURE_COMMAND
body: <p>I'm trying to add an Automake project as an external project.
  The Automake configure script (usually run with <code>./configure</code>) contains a relative path to a resource file. The file is found when I run configure manually, because my working directory is in the source directory. However, when I run <code>configure</code> with the <code>ExternalProject_Add</code>, it can't find the resource file because the working directory is <code>CMAKE_CURRENT_BINARY_DIR</code>.</p>
  
  <pre><code>ExternalProject_Add(zlib
          SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}
          CONFIGURE_COMMAND sh ${CMAKE_CURRENT_SOURCE_DIR}/configure
          BUILD_COMMAND make)
  </code></pre>
  
  <p>How can I set the working directory for the configuration step so that the config script finds the required files?</p>