title: Install libcurl4-openssl-dev using CMake's External Project
body: <p>Currently I have this bit of code in one of my projects:</p>
  
  <pre><code>find_package(CURL REQUIRED)
  if(${CURL_FOUND})
  else(${CURL_FOUND})
      message(STATUS "Could not find libcURL.  This dependency will be downloaded.")
      ExternalProject_Add(
          libcurl
          GIT_REPOSITORY "git://github.com/bagder/curl.git"
          GIT_TAG "1b6bc02fb926403f04061721f9159e9887202a96"
          SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/lib/curl
          PATCH_COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/lib/cURL/buildconf
          CONFIGURE_COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/lib/cURL/configure --prefix=&lt;INSTALL_DIR&gt;
          BUILD_COMMAND ${MAKE}
          UPDATE_COMMAND ""
          INSTALL_COMMAND ""
          LOG_DOWNLOAD ON
          LOG_UPDATE ON
          LOG_CONFIGURE ON
          LOG_BUILD ON
          LOG_TEST ON
          LOG_INSTALL ON
      )
      ExternalProject_Get_Property(libcurl source_dir)
      ExternalProject_Get_Property(libcurl binary_dir)
      set(CURL_SOURCE_DIR ${source_dir})
      set(CURL_BINARY_DIR ${binary_dir})
      set(CURL_LIBRARIES ${CURL_BINARY_DIR}/lib/.libs/libcurl.dylib)
      include_directories(${CURL_SOURCE_DIR})
      set(DEPENDENCIES ${DEPENDENCIES} libcurl)
  endif(${CURL_FOUND})
  </code></pre>
  
  <p>One of the main priorities of the project is to be as easy as possible to install for an end user while still compiling from scratch. One error I'm running into with this bit of code is the following error gets generated when running CMake:</p>
  
  <pre><code>CMake Error at /usr/share/cmake-2.8/Modules/FindPackageHandleStandardArgs.cmake:97 (MESSAGE):
    Could NOT find CURL (missing: CURL_LIBRARY CURL_INCLUDE_DIR)
  Call Stack (most recent call first):
    /usr/share/cmake-2.8/Modules/FindPackageHandleStandardArgs.cmake:288 (_FPHSA_FAILURE_MESSAGE)
    /usr/share/cmake-2.8/Modules/FindCURL.cmake:52 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
    CMakeLists.txt:29 (find_package)
  </code></pre>
  
  <p>This error occurs because <code>libcurl4-openssl-dev</code> is not installed on the system where CMake is running.  I was wondering how I might go about installing this dependency using CMake.  Any suggestions?</p>