reference: move leap second source into leapdb

Separate out source of leap second data into a new module in preparation
for supporting more sources such as leap-seconds.list.

Makefile.in

@@ -37,7 +37,7 @@ GETDATE_CFLAGS = @GETDATE_CFLAGS@
 
 EXTRA_OBJS = @EXTRA_OBJS@
 
-OBJS = array.o cmdparse.o conf.o local.o logging.o main.o memory.o quantiles.o \
+OBJS = array.o cmdparse.o conf.o leapdb.o local.o logging.o main.o memory.o quantiles.o \
        reference.o regress.o rtc.o samplefilt.o sched.o socket.o sources.o sourcestats.o \
        stubs.o smooth.o sys.o sys_null.o tempcomp.o util.o $(EXTRA_OBJS)