I found a Makefile decribed in a Calendar document and tried it out and it seems to work (with a bit of tinkering).
The document is at:http://docs.sun.com/source/816-6712-10/introens.html
I'm able to compile apub.c and asub.c. I haven't tested whether they run corectly. apub does return a message about the required options.
Here's my slightly altered verson:
#
# Sample makefile
#
# your C compiler
CC = gcc
# LIBS
BASE = /opt/SUNWics5/cal
LIBS = -L$(BASE)/bin -lens -lgap -lxenp -lcyrus -lchartable -lyasr -lasync -lnspr3 -lplc3 -lplds3 -R$(BASE)/bin
all: apub asub
apub: apub.c
$(CC) -o apub -I $(BASE)/csapi/include apub.c $(LIBS)
asub: asub.c
$(CC) -o asub -I $(BASE)/csapi/include asub.c $(LIBS)