How can I call functions in a different C source file from a Perl XS module?

This is actually rather straightforward:

hello.h

#ifndef H_HELLO
const char *hello(void);
#define H_HELLO
#endif

hello.c

const char *
hello(void) {
    return "Hello";
}

Example.xs

#define PERL_NO_GET_CONTEXT
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "ppport.h"

#include "hello.h"

MODULE = My::Example        PACKAGE = My::Example       PREFIX = MY_

PROTOTYPES: DISABLE

const char *
MY_hello()
    CODE:
        RETVAL = hello();
    OUTPUT:
        RETVAL

t/My-Example.t

use strict;
use warnings;

use Test::More;
BEGIN { use_ok('My::Example') };

is(My::Example::hello(), 'Hello', 'hello returns "Hello"');

done_testing;

[~/tmp/My-Example]$ prove -vb t/My-Example.t
t/My-Example.t ..
ok 1 - use My::Example;
ok 2 - hello returns "Hello"
1..2
ok
All tests successful.
Files=1, Tests=2,  0 wallclock secs 
( 0.04 usr  0.01 sys +  0.03 cusr  0.00 csys =  0.08 CPU)
Result: PASS

The Makefile.PL was generated by h2xs. The only thing I changed was to uncomment the following line:

# Un-comment this if you add C files to link with later:
 OBJECT => '$(O_FILES)', # link all the C files too