cpperror.c (cpp_file_line_for_message): If 'line' is zero, just print "<command line>".
* cpperror.c (cpp_file_line_for_message): If 'line' is zero,
just print "<command line>". If 'filename' is null or an
empty string, print "<stdin>" for the filename.
* cpplib.c (do_define): Don't print the 'location of the
previous definition' message if we're still parsing the
command line.
(cpp_pedwarn_with_file_and_line): Always call
cpp_file_line_for_message.