title: Unsupported protocol while download tar.gz package
body: <p>I have just upgrade my CMake from version 2.8 to 3.2.</p>
  
  <p>It's working like a charm in CMake 2.8 but, after the upgrade, it's failing.</p>
  
  <p>I'm trying to build third party library using <code>ExternalProject_Add()</code> CMake function.</p>
  
  <pre><code>ExternalProject_Add(
    luacov
    URL https://github.com/keplerproject/luacov/archive/v0.7.tar.gz
    DOWNLOAD_DIR ${EXTERNAL_PROJECT_DOWNLOAD_DIR}
    CMAKE_ARGS -DCMAKE_TOOLCHAIN_FILE=${MY_TOOLCHAIN_FILE}
    SOURCE_DIR ${EXTERNAL_PROJECT_SRC_DIR}/luacov
    BINARY_DIR ${EXTERNAL_PROJECT_BUILD_DIR}/luacov
    UPDATE_COMMAND ""
    PATCH_COMMAND ""
  )
  </code></pre>
  
  <p><strong>MY OBSERVATION:</strong></p>
  
  <ol>
  <li>Using <code>GIT_REPOSITORY</code> option, <code>ExternalProject_Add()</code> allow <code>http</code> and <code>https</code> protocol to download external project.</li>
  <li>Using <code>URL</code> option, <code>ExternalProject_Add()</code> only allow <code>http</code>, but not <code>https</code> protocol to download external project.</li>
  </ol>
  
  <p><strong>PROBLEM:</strong></p>
  
  <p>Is there any way to download and build external project using <code>https</code> protocol? </p>
  
  <p><strong>ERROR:</strong></p>
  
  <pre><code>[ 16%] Performing download step (download, verify and extract) for 'luacov'
  -- downloading...
   src='https://github.com/keplerproject/luacov/archive/v0.7.tar.gz'
   dst='/home/build/my_build/external_projects/downloads/v0.7.tar.gz'
   timeout='none'
  CMake Error at /home/build/my_build/luacov-prefix/src/luacov-stamp/download-luacov.cmake:21 (message):
   error: downloading
  'https://github.com/keplerproject/luacov/archive/v0.7.tar.gz' failed
  
  status_code: 1
  status_string: "Unsupported protocol"
  log: Protocol "https" not supported or disabled in libcurl
  
  Closing connection -1
  
  make[3]: *** [luacov-prefix/src/luacov-stamp/luacov-download] Error 1
  make[2]: *** [CMakeFiles/luacov.dir/all] Error 2
  make[1]: *** [CMakeFiles/luacov.dir/rule] Error 2
  make: *** [luacov] Error 2
  </code></pre>