Exit with an error if we can't find libvarnishapi, instead of
just warning about missing macros.

This is useful for changing all references of libvmod-example/example
into your new vmod name.

One-off task for all vmods, but if you write more than a couple you
quickly get fed up with this drudgery.

Conflicts:
	configure.ac

Make man page generation generic

The creation of vmod_example.3 man page from README.rst was previously
hard-coded. It is now possible to generate any man page against any rst
template. Man pages are now ignored, instead of leaving the repository
in a dirty state.