Fix "undefined symbol: luaI_openlib" errors

Fix "undefined symbol: luaI_openlib" errors
when using slnunicode with lua 5.1.5 (in OpenResty 1.7.0.1)

From the comment in CMakeLists.txt, this might break 5.0 compatibility,
but I couldn't get it to run without these changes in 5.1.5.