Avoid the use of the non-portable -D argument to ls.

This was used to store the mtime of the source file in a commment in a
generated header file.  This is of little-to-no diagnostic value and
the result doesn't even end up in the source tree.

Reported by:	arichardson
Reviewed by:	arichardson
MFC after:	1 days
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D21605

lib/libbsnmp/libbsnmp/Makefile

@@ -26,9 +26,7 @@ MAN=	asn1.3 bsnmpagent.3 bsnmpclient.3 bsnmplib.3
 
 snmptc.h : tc.def
 	(\
-	echo -n "/* autogenerated from tc.def; ";\
-	ls -l -D "%F %T" ${.ALLSRC} | awk '{printf("%s %s", $$6, $$7)}';\
-	echo "*/";\
+	echo "/* autogenerated from tc.def */";\
 	echo "#ifndef snmptc_h_1529923773";\
 	echo "#define snmptc_h_1529923773";\
 	gensnmptree -E -f <${.ALLSRC};\