Support for current varnish master / varnish 6

new now need the VARNISHSRC configure variable again pointing to the
varnish source tree to check the workspace_client parameter setting.

Regression tested with make check against:

varnish-5.2.1
varnish-5.1.0
varnish-4.1.9
varnish-3.0.7
varnish-2.1.5