title: cmake: ExternalProject_Add() does not apply cmake arguments
body: <p>I have troubles while building an external library (glfw) in my cmake file:</p>
  <pre><code>[...]
  
  ExternalProject_Add(glfw
      PREFIX &quot;${CMAKE_CURRENT_SOURCE_DIR}/glfw&quot;
      URL &quot;https://github.com/glfw/glfw/releases/download/3.2.1/glfw-3.2.1.zip&quot;
      URL_HASH SHA256=b7d55e13e07095119e7d5f6792586dd0849c9fcdd867d49a4a5ac31f982f7326
      CMAKE_ARGS &quot;-DGLFW_BUILD_EXAMPLES=OFF -DGLFW_BUILD_TESTS=OFF&quot;
      INSTALL_COMMAND &quot;&quot;
      TEST_COMMAND &quot;&quot;
  )
  
  add_executable(test main.cpp)
  add_dependencies(test glfw)
  </code></pre>
  <p>Although this works quite well, the given cmake arguments <code>-DGLFW_BUILD_EXAMPLES=OFF -DGLFW_BUILD_TESTS=OFF</code> are ignored.</p>
  <p>According to the <a href="https://www.glfw.org/docs/latest/compile.html" rel="nofollow noreferrer">glfw documentation</a> the arguments should disable building examples and tests. I only want to build the library (target: glfw).</p>
  <p>If I try to build the library separately</p>
  <pre><code>cmake -DGLFW_BUILD_EXAMPLES=OFF -DGLFW_BUILD_TESTS=OFF
  </code></pre>
  <p>then it works as expected.</p>
  <p>I also study the <a href="https://cmake.org/cmake/help/latest/module/ExternalProject.html" rel="nofollow noreferrer">cmake documentation</a> but I can't get any further.</p>
  <p>Am I doing something wrong? Can you choose which target to build or not?</p>