Spotted by Martin, when a VMOD relying on varnish.m4 is built with
--prefix in the configure command-line, a pseudo hard-coded vmoddir
is assumed:

    $libdir/varnish/vmods

Instead, it now asks pkg-config for the vmoddir relative to the
libdir. It worked in all cases with upstream Varnish Cache, but
downstream modifications of the vmoddir would then be ignored.