title: Luarocks on windows not recognizing my lua_libdir
body: <p>i want to install nngraph on lua using luarocks 
  using this code</p>
  
  <pre><code>luarocks --from=https://raw.githubusercontent.com/torch/rocks/master/ install nngraph
  </code></pre>
  
  <p>but it's give me an error 
  it said :</p>
  
  <blockquote>
    <p>Error: Failed finding Lua library. You may need to configure LUA_LIBDIR.</p>
  </blockquote>
  
  <p>does anyone have same experience? can you solve it ?</p>
  
  <p>this was my config-5.2.lua :</p>
  
  <pre><code>rocks_trees = {
      { name = [[user]],
           root    = home..[[/luarocks]],
      },
      { name = [[system]],
           root    = [[d:\shared\ta\_bootstrap\_install\]],
      },
  }
  variables = {
      MSVCRT = 'MSVCRT',
      LUALIB = 'D:\\Shared\\TA\\_bootstrap\\_install\\lib\\liblua.dll.a',
      LUA_LIBDIR = 'D:\\Shared\\TA\\_bootstrap\\_install\\lib'
  }
  verbose = false   -- set to 'true' to enable verbose output
  </code></pre>