lib/Makefile.am: Change how libsyslog-ng.so is versioned

Instead of using the current VERSION for the soname of the library, use
the first two parts of it as a library major version, and then use
libtool's versioning scheme. Comments explaining how that works were
added to lib/Makefile.am. This fixes #190.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>