Fix vmoddir when using --prefix at configure time

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.

varnish.m4

@@ -95,7 +95,8 @@ AC_DEFUN([_VARNISH_VMOD_CONFIG], [
 		[NONE], [
 			vmoddir=$VARNISHAPI_VMODDIR
 			ac_default_prefix=$VARNISHAPI_PREFIX],
-		[vmoddir=$libdir/varnish/vmods]
+		[vmoddir=$($PKG_CONFIG --define-variable=libdir=$libdir \
+			--variable=vmoddir varnishapi)]
 	)
 	AC_SUBST([vmoddir])