disable the rule to copy resize.c since it fails on r/o src trees:

cp: /home/source/ab/HEAD/src/usr.bin/resize/resize.c: Permission denied

usr.bin/resize/Makefile

@@ -1,4 +1,4 @@
-#	$NetBSD: Makefile,v 1.5 2021/02/28 00:43:24 christos Exp $
+#	$NetBSD: Makefile,v 1.6 2021/02/28 09:04:28 mrg Exp $
 
 .include <bsd.own.mk>
 
@@ -13,6 +13,8 @@ LDADD+= -lutil
 
 XTERM=${X11SRCDIR}/external/mit/xterm/dist
 
+# XXX disable for now; breaks on r/o source trees.
+.if 0
 .if exists(${XTERM})
 .for i in resize.c resize.man
 copy:: ${.CURDIR}/${i}
@@ -20,6 +22,7 @@ ${.CURDIR}/${i}: ${XTERM}/${i}
 	cp -p ${.ALLSRC} ${.TARGET}
 .endfor
 .endif
+.endif
 
 CLEANFILES+=resize.1