Support -DLUA_DIR flag

Setting the environment variable LUA_DIR for a command in Windows is a PITA. This adds support for defining the location of the LUA_DIR from the command-line using Cmake's -D flag.

The effects using bash syntax are:

    cmake .                           # LUA_DIR is undefined
    LUA_DIR=foo cmake .               # LUA_DIR=foo
    cmake -DLUA_DIR=bar .             # LUA_DIR=bar
    LUA_DIR=foo cmake -DLUA_DIR=bar . # LUA_DIR=bar