Support for linking with LuaJIT

Thread safety is lost if you enable LuaJIT with --enable-luajit
Unused variable removed
Thread guard function added in case of link with LuaJIT; thread test added
OASIS option stub_debug renamed into stubdebug for compatibility with GODI
Merge branch 'luajit'

Conflicts:
	src/lua_api_lib.ml
	src/lua_api_lib.mli
[ISSUE #3] The package configure itself to find the correct library names