Enabled popen on OSX to enable unit testing via "busted"

Hi,

Unit testing via "busted" was not working on OSX before because io.popen was not enabled. To reproduce the bug, simply run the sample in moai-dev/samples/test/popen on OSX. The sample bails out indicating that 'popen' is not supported.

This happens because no compilation flag was included to enable popen. See 3rdparty/lua-5.1.3/src/luaconfig.h:668. The fix is to add the LUA_USE_POPEN when compiling on OSX.

Actually, it appears that on OSX, the proper flag to include is LUA_USE_MACOSX, which will also enable a number of other flags. I wasn't sure if the exclusion of the LUA_USE_MACOSX flag was by design, so the fix was made by adding LUA_USE_POPEN instead.