You build them. Or if the latest compilers don't build against the older glibc (which is rare in my experience, but may be possible), then you just don't suppor

You build them. Or if the latest compilers don't build against the older glibc (which is rare in my experience, but may be possible), then you just don't support the older glibc any more. But I've never had trouble building against older glibcs.