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

see f115bcad262c5464dceafe7fd616963b7a0b3f4f

For varnish2 itself, we could keep the const qualifyer, but we use
the same interface for varnish3 and the varnish3 vmod interface
mandates an un-const argument. Special-casing this for varnish2
has no actual benefit.

This fixes compilation for key

	sogou*!sogoumobilebrowser

- configure by pkg-config (new varnish4 / master standard - see vmod-example)
- make distcheck works now (working around dependency tracking issues with
  --disable-dependency-tracking )
- split out checks into a seperate subtree to ensure the build is complete
  before checking (parallel make wasn't working otherwise)

dont truncate the last character

when called below in

	if (ua_prepend[i])
		appnd(w, space, ua_prepend[i], l);

appnd would exand such that only the first line (l = strlen(r))
was conditional on ua_prepend[i] being non-null, which then would
trigger a null pointer dereference in strncpy.

This is a particularly embarassing bug.