Provide more flexible version numbers to the code

Having the individual parts in the code allows us to build version
numbers like "1.2" while leaving "3" out in a clean fashion, that is
without regular expressions or the like. This might be used together
with configuration format versions.

Why m4 code? AM_INIT_AUTOMAKE, which could take a shell variable, is
considered deprecated[1] and should be replaced by AC_INIT. Unfortunately,
AC_INIT is expanded at build time, so one has to use m4 to build
composite values like this version number[2].

[1] http://www.gnu.org/software/libtool/manual/automake/Public-macros.html
[2] http://www.mail-archive.com/autoconf@gnu.org/msg16720.html

Reviewed-by: iustinp