Requiring autoconf-archive is the right way.

Ref: https://github.com/varnishcache/varnish-cache/commit/5450550906d0809dfc21ae9d3a5b14446c1809d4

ref: 0c74d1b99f1f43fc99725fe14322760afa7858ee

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.