Add pidfile() from NetBSD/OpenBSD, missing in GLIBC.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

Makefile

@@ -56,7 +56,7 @@ endif
 DISTFILES   = README LICENSE
 HEADERS     = lite.h
 OBJS       := chomp.o copyfile.o dir.o fexist.o fisdir.o fmode.o fsendfile.o
-OBJS       += makepath.o rsync.o strlcpy.o strlcat.o strtonum.o tempfile.o
+OBJS       += makepath.o pidfile.o rsync.o strlcpy.o strlcat.o strtonum.o tempfile.o
 DEPS       := $(OBJS:.o=.d)
 JUNK        = *~ *.bak *.map .*.d *.d DEADJOE semantic.cache *.gdb *.elf core core.*