Check version number consistency on updates

[mhvk]

When we moved to meson for building by default, we updated the release instructions to change version numbers in meson.build. But we still also have to make changes in configure.ac, since that way of building is also supported, and in fact is what is used by pyerfa.

I now updated the release instructions to reflect this, but, really, it would be nicer if there was just one place!

Any ideas?

p.s. If not, we probably should at least have a CI run that checks that everything is consistent.