title: cmake ExternalProject_Add how to download URL non-tar file?
body: <p>How does one set up an External project to download a link that isn't a .tgz file?</p>
  
  <p>For example <a href="https://github.com/philsquared/Catch" rel="nofollow noreferrer">Catch</a> provides a release that is a single header distribution.  I would like to just download this rather than the git repo or the .tgz release.  But I haven't figured out how to tell CMake to do this.</p>
  
  <p>I would like to do something like:</p>
  
  <pre><code>Include(ExternalProject)
  ExternalProject_Add(
      catch
      PREFIX ${CMAKE_BINARY_DIR}/catch
      URL https://github.com/philsquared/Catch/releases/download/v1.9.6/catch.hpp ${CMAKE_BINARY_DIR}/catch
      CONFIGURE_COMMAND ""
      BUILD_COMMAND ""
      INSTALL_COMMAND ""
      LOG_DOWNLOAD ON
  )
  </code></pre>
  
  <p>This fails as URL assumes that url is a compressed tar file.  I have tried various variants of the <strong>DOWNLOAD_COMMAND</strong> without any success either.</p>