C :: Creating A Library Function

I want to create a C library function that i can directly call in my code from any .c file having main program.following are codes...code of library function "foo.c"

Code:

#include "foo.h"
int foo(int x) /* Function definition */ {
return x + 5;
} header file "foo.h"

Code:

#ifndef FOO_H_ /* Include guard */
#define FOO_H_
int foo(int x); /* An example function declaration */
}

[code]....

to use this i have to compile the file in below manner...

Code: gcc -o my_app main.c foo.c

My concern here is that i want to compile the main.c and use function without compiling foo.c with i.e.

Code: gcc -o my_app main.c

any user of this function should only compile his program and should be able to use the function, the foo.c file should remain hidden from him

my system is Linux 2.6.18-308.4.1.el5 #1 SMP Wed Mar 28 01:54:56 EDT 2012 x86_64 x86_64 x86_64 GNU/Linux