Numbers are arbitrary, and we've never successfully made it to
version 1.0 after like 15 years or something.

Furthermore, plymouth has a very slow release schedule at present
and some distros hate building from git.

So, I'd like to start generating tarballs more regularly.

Adopting a version number derived from the date will help facilitate
that.

This commit changes AC_INIT to compute the version automatically.

#143
#158