title: How can I make CMake download an external package and use it?
body: <p>CMakeLists.txt:</p>
  
  <pre><code>cmake_minimum_required(VERSION 3.3)
  project(CMakeTest)
  set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
  find_package(event-cmake REQUIRED)
  
  file(GLOB SOURCES "*.cpp" )
  
  add_executable(test ${SOURCES})
  </code></pre>
  
  <p>cmake/Findevent-cmake.cmake:</p>
  
  <pre><code>ExternalProject_Add(event-cmake
    GIT_REPOSITORY https://github.com/libevent/libevent.git
    UPDATE_COMMAND ""
    INSTALL_COMMAND ""
  )
  </code></pre>
  
  <p>I know that the CMakeLists here has no chance to include the resolved package, but I cannot even get CMake to download the external repo. It errors out with:</p>
  
  <pre><code>CMake Error at cmake/Findevent-cmake.cmake:3 (ExternalProject_Add):
    Unknown CMake command "ExternalProject_Add".
  Call Stack (most recent call first):
    CMakeLists.txt:4 (find_package)
  </code></pre>
  
  <p>Is there a way to make it so that cmake will download the project and link to it?</p>