[oe] Avoid version going backwards for beta versions

[Andreas Müller]

Hi,

There was a pre-release for fluidsynth-2.0.0.beta and I expect the
release will be fluidsynth-2.0.0. How would I name the beta recipe to
avoid version going backwards with 2.0.0?

I know there was a thread where Martin gave hints how to achieve but I
can't find it - sigh.

Please help a forgetful old man :)

Andreas