building a c function

Hello all,

I know this is a very elementary question, but my excuse
is I have not programmed in C or written makefiles for
15+ years...

I am trying to write a C-language function, call it my_func.
But I also have a my_subs.o that does the heavy lifting
for my_func.c.  My makefile is:

MODULES = my_func
PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)
OBJS = my_subs.o    # Is this right?

But I see no indication in the output that my_subs.o is
being used:

  gcc -O2 -g ... -fpic -shared -o my_func.so my_func.o

How do I tell the makefile to include my_subs.o when building
the shared lib?