readme-alpha option and .90 pretests

[Karl Berry]

Hello Alexandre and all,

rms has decreed that pretest version numbers, say for a forthcoming
version 4.6, should have the form 4.5.90, or the form 4.6.YYYYMMDD.  See
the Test Releases node of maintain.texi.

So, for automake's purposes, I suggest that the readme-alpha option only
consider the release an "alpha" release if the tertiary version is >90,
not simply if it exists.  

Did anyone ever make a test release with a version number like 4.5.1?  I
don't think so.  And many people make real releases with three-part
version numbers.

(I noticed this because the recent mailutils 0.3.1 release contained the
README-alpha file; I already reported that to Alain ...)

All the best,
karl