how to not include certain functions in a header file for use in C programing

To paint the picture let's imagine we have a header called headertest.h and where going to use it in a file called test.c

It's context look like this

#include <stddef.h>extern size_t string_lenght(const char *);size_t string_lenght(const char *str){    size_t n = 0;    while (str[n] != '\0')        n++;    return (n);}

It's purpose is to count the lenght of a given string and give output as size_t

if we include it in our test.c file like this and compile with gcc -o test test.c

#include "<location to header>/headertest.h"int main (){}

it compiles successfully and by running the command du -b test | awk '{print $1}' it outputs 16504, telling us it's size is 16504 bytes. However if we comment #include and just leave int main () and recompile test and run the same command again, we get 16464.

so in conclusion, how do you tell C to specifically only include a function when compiling only if it's present in the main C program, from a self made header file.