build: Tie interface age to the development cycle

build: Tie interface age to the development cycle

We don't want to increase the interface age manually, because we're
going to end up forgetting about it.

Instead, we should tie it to the rest of the version:

 - in stable (even minor) cycles, we don't add new API; the interface
   age is the same as the micro version
 - in unstable (odd minor) cycles, every new release might have new API,
   or updates to newly added API; keep the interface age to 0

This removes one more manual thing to change during release, and keeps
us honest with our promise not to add symbols during stable cycles.