Fix for br970 - iverilog silently eats warnings when comma present in -W flag.

The -W option does not (currently) support comma separated lists and
silently ignores any string that doesn't match a known warning class.
Fix by outputing a warning message when the -W argument is unknown.

driver/main.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000-2014 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 2000-2015 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -568,6 +568,9 @@ static void process_warning_switch(const char*name)
 		  cp[0] = cp[1];
 		  cp += 1;
 	    }
+      } else {
+	    fprintf(stderr, "Ignoring unknown warning class "
+		    "%s\n", name);
       }
 }