Fix deprecated use of zeek-config --caf_root.

This flag became deprecated with zee-5.0.0 and went away some time
after. Only use it if `zeek-config` actually still supports the flag.

features/plugin/configure

@@ -158,7 +158,14 @@ if [ -z "$zeekdist" ]; then
     fi
 
     if [ -z "$caf_root" ]; then
-        append_cache_entry CAF_ROOT_DIR PATH "$(${zeek_config} --caf_root)"
+        # The `--caf_root` flag became deprecated with zeek-5.0.0 and was
+        # removed some time after. Only use it if `zeek-config` actually still
+        # supports it.
+        #
+        # TODO: Remove this whole block once support for <=zeek-4.x.x is dropped.
+        if zeek-config --caf_root >/dev/null 2>&1; then
+            append_cache_entry CAF_ROOT_DIR PATH "$(${zeek_config} --caf_root)"
+        fi
     fi
 else
     if [ ! -e "$zeekdist/zeek-path-dev.in" ]; then