Fixes #5

The Varnish version appears only once, making it hard to break on
Varnish upgrades.

Although it may build with 4.1.2+, stick to the branching scheme.

Libtool itself requires most of the compilation toolchain, so we don't
need to duplicate effort.

Make it choke on warnings, set up the test driver and use silent rules.

It is now replaced by AC_CANONICAL_TARGET but regardless, the $target
variable isn't used anywhere.

Require a more recent autoconf, and name the dist archive vmod-example
to match the usual package naming scheme. Update the copyright notice.