recognize but ignore -undef flag

util/cpp/cpp.6

@@ -58,6 +58,9 @@ when generating makefile dependencies, generate them in the following format:
 .IP ""
 where "file.o" is derived from the source file name. Normally, only a list
 of files included is generated.
+.IP -\fBundef\fR
+.br
+this flag is silently ignored, for compatibility with other preprocessors.
 .PP
 The following names are always available unless undefined:
 .IP __FILE__

util/cpp/options.c

@@ -31,6 +31,12 @@ do_option(text)
 	case 'x':
 		options[*text] = 1;
 		break;
+	case 'u':
+		if (! strcmp(text,"ndef")) {
+			/* ignore -undef */
+			break;
+		}
+		/* fall through */
 	default:
 		error("illegal option: %c", text[-1]);
 		break;