Improve autoconf robustness.
Improve autoconf robustness.

from libvmod-example, changes originally made by Nils Goroll.

wjordan/libvmod-cookie,lkarsten/libvmod-cookie,ruckserve/libvmod-cookie,lkarsten/libvmod-cookie,tricycle/libvmod-cookie,varnish/libvmod-cookie,wjordan/libvmod-cookie,ruckserve/libvmod-cookie,tricycle/libvmod-cookie,varnish/libvmod-cookie