Add Doxygen config statements for newer versions

Newer versions of Doxygen (1.14 in Arch Linux) default HAVE_DOT and
CALL_GRAPH to NO, although older versions also default the latter to NO
as well.

Document why DISTRIBUTE_GROUP_DOC was added.  Removes a bit of
redundancy.

doc/hamlib.cfg.in

@@ -61,7 +61,11 @@ ENABLED_SECTIONS      = ""
 PREDEFINED            = DOXYGEN "DOC_HIDDEN"
 JAVADOC_AUTOBRIEF     = NO
 OPTIMIZE_OUTPUT_FOR_C = YES
+# Allow anonymous group for related macro definitions
 DISTRIBUTE_GROUP_DOC  = YES
+# Later versions default to NO
+HAVE_DOT              = YES
+CALL_GRAPH            = YES
 
 MAN_LINKS             = NO
 MACRO_EXPANSION       = YES