When you supply an existing Lua installation, the suggested value for LUA_CPATH is wrong.

```sh
install /P c:\LuaRocks /LUA d:\trunk_git\packages\Lua5.1 /F

(...)

 You may want to add the following elements to your paths;
PATH     :   d:\trunk_git\packages\Lua5.1\bin\;c:\LuaRocks\2.1
LUA_PATH :
c:\LuaRocks\share\lua\5.1\?.lua;c:\LuaRocks\share\lua\5.1\?\init.lua
LUA_CPATH:   d:\trunk_git\packages\Lua5.1\lib\lua\5.1\?.dll
```

Even `luarocks path` shows the correct values:

```sh
SET LUA_CPATH=c:\LuaRocks/lib/lua/5.1/?.dll;c:\LuaRocks\lib\lua\5.1\?.dll
```
Merge pull request #152 from ignacio/install_wrong_cpath

When you supply an existing Lua installation, the suggested value for LU...