Use the -F flag of install(1) to ensure the file's content is flushed to disk.

OK deraadt

src/etc/rc

@@ -1,4 +1,4 @@
-#	$OpenBSD: rc,v 1.479 2016/05/14 08:21:40 jmc Exp $
+#	$OpenBSD: rc,v 1.480 2016/05/22 15:16:47 rpe Exp $
 
 # System startup script run by init on autoboot or after single-user.
 # Output and error are redirected to console by init, and the console is the
@@ -181,7 +181,7 @@ rebuildlibs() {
 			cc -shared -o $_lib $(ls *.so | sort -R) $(cat .ldadd)
 			[[ -s $_lib ]] && file $_lib | fgrep -q 'shared object'
 			LD_BIND_NOW=1 LD_LIBRARY_PATH=$_tmpdir awk 'BEGIN {exit 0}'
-			install -S -o root -g bin -m 0444 $_lib /usr/lib/$_lib
+			install -F -S -o root -g bin -m 0444 $_lib /usr/lib/$_lib
 		)
 		rm -rf /tmp/_librebuild.${_tmpdir#*.}
 	done