make pidfile writing configurable.

[ui]toa_to_buf: change API. No users yet.

libbb/Kbuild

@@ -61,6 +61,7 @@ lib-y += perror_msg.o
 lib-y += perror_msg_and_die.o
 lib-y += perror_nomsg.o
 lib-y += perror_nomsg_and_die.o
+lib-y += pidfile.o
 lib-y += process_escape_sequence.o
 lib-y += procps.o
 lib-y += read.o