[BSDfan666]

The warning messages printed at compile time, are not a GCC modification, it's simply a frequently unused feature of the GNU linker.

#include <stdio.h>

__warn_references(printf, "Are you using printf????? Yay!!");

int
main(int argc, char **argv) {
	printf("This is a simplistic, unusual program.\n");
}

Results:

$ gcc example.c -o example
/tmp//ccw21238.o(.text+0x21): In function `main':
: Are you using printf????? Yay!!
$

The function is defined in <cdefs.h>, which is pulled in by <stdio.h>.