This removes the need for non-VRT VMODs to require a Varnish source tree
to build, especially since varnish.m4 only supports Varnish installations
today and would leave VMOD authors on their own in order to build from
a source tree. Since $ABI defaults to strict, any VMOD author actively
choosing vrt compliance is responsible for ensuring it only uses blessed
symbols.

In our official packages, they land in the varnish-dev deb and the
varnish-devel RPM:

    $ rpm --query --package --list varnish-trunk-1.el7.x86_64.rpm |
    > grep cache_varnishd.h

    $ rpm --query --package --list varnish-devel-trunk-1.el7.x86_64.rpm |
    > grep cache_varnishd.h
    /usr/include/varnish/cache/cache_varnishd.h

You will have to trust me for the Debs as it's a bit cumbersome for me to
build them locally.