Depending on glibc can be portable enough if you build against the oldest glibc version that you need. Running on newer systems works.

How do you get the latest toolchain and compiler but an old glibc? The tools have their own dependency on libc

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.