title: Unable to find Lua headers with find_package in cmake
body: <p>I'm trying to use CMake to build generate the make file for a project of mine that uses Lua. When I run make I get this error:</p>
  
  <pre><code>/path/to/my/project/luaudio/luaudio.c:1:17: fatal error: lua.h: No such file or directory
  </code></pre>
  
  <p>In the CMakeLists.txt file, I have the following lines, which I thought would do it, but apparently they're not enough:</p>
  
  <pre><code>find_package(Lua51 REQUIRED) 
  set(Luaudio_INCLUDE_DIRS ${Luaudio_SOURCE_DIR} ${Lua51_INCLUDE_DIRS} PARENT_SCOPE)
  include_directories(${Luaudio_INCLUDE_DIRS})
  </code></pre>
  
  <p>Lua51_Include_Dirs appears to be empty (attempting to run it though the message command doesn't print anything) so I suspect that it just can't find it. Do I need to specify where to look for Lua? I was under the impression that the whole point of find_package was that it would look in a set a predefined places so that I don't need to specify where it is specifically. </p>
  
  <p>(This is on an Ubuntu machine and I do have the Lua packages installed.)</p>