[meson] update generated headers only when the result is different

This way it won't ruin incremental builds.

We need a better way to declare source altering tasks
https://github.com/mesonbuild/meson/issues/7156
yet this is good enough despite being not idiomatic.

It is however not that smooth yet as the change may is detected on the
next meson run. One of course can issue ./gen-ragel-artifacts.py
manually for better experience before running meson.