linking assembly object file with C object file on OS X and can't find symbol - macos

Question

I have a library defined in libadd.asm, it exposes one "function" _add. I have a .c source file that refers to add and I'm trying to get the two object files to link, but am encountering this error regardless of the order in which I link the object files:
Undefined symbols for architecture x86_64:
"_add", referenced from:
_main in prog.o
Here's the code:
// prog.c
#include <stdio.h>
int add();
int main() {
printf("%d\n", add(4, 5));
return 0;
}
And here's the assembly file. It almost certainly doesn't respect the appropriate calling convention. I don't really understand what I should be doing to shuffle the values between registers. (That's what I was trying to figure out originally.)
; libadd.asm
_add:
add eax, edx
ret
Here's what I'm using to the tiny project. I'm intentionally shadowing the implicit .c.o rule with one that does as little as possible and ignores *FLAGS. I'm using cc to drive the linker because that's the simplest way I know to link in the c runtime/standard library/whatever it's called. I've always tried linking with prog.o and libadd.o in the other order.
all: prog
prog: prog.o libadd.o
$(CC) -o prog $^
%.o: %.asm
nasm -f macho64 -o $# $<
%.o: %.c
$(CC) -c -o $# $<
clean:
$(RM) $(wildcard *.o)
running make produces the following output
cc -c -o prog.o prog.c
nasm -f macho64 -o libadd.o libadd.asm
cc -o prog prog.o libadd.o
Undefined symbols for architecture x86_64:
"_add", referenced from:
_main in prog.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [prog] Error 1
Exit 2
libadd.o gets assembled successfully and appears to have the right symbol in it.
% nm libadd.o
0000000000000000 t _add
Why is ld complaining that it can't find the symbol?