Simplified version number (redux)

From now on, AVIM versions take the form *.X.Y.Z, where X is a major version number, Y is a minor version number, and Z is a build number. The * doesn’t mean anything; it’s part of the version string mainly to fool Mozilla Add-ons’ version comparison. The first such version string is *.5.3.0, chosen to look newer than 20080728.526b1.

Fixes #95.
Git tags can’t contain *
so use v instead and transform it into *. before building.