Use moonjit 2.1.2 as base tarball
    
    The LuaJIT project has not been doing releases for a few years, so use
    releases from the v2.1 branch of moonjit as the base tarball.
    
    The version number follows moonjit's numbering.  This is a risk if
    upstream LuaJIT suddenly starts doing releases, but it is not serious
    enough since the moonjit v2.1 branch remains in sync.  We can mitigate
    the version number inconsistency by slowing down moonjit version
    number bumps.
    
    I have also dropped changes that are unlikely to make it into LuaJIT.
    I will propose a separate package for moonjit v2.2 and beyond since
    that will have a very different trajectory from LuaJIT.