foo.c

#include <stdio.h>
#include "foo.h"

void foo(void) {
    printf("foo!\n");
}

foo.h

#ifndef FOO_H
#define FOO_H

void foo(void);

#endif /* FOO_H */

foo_shim.c

#include <stdio.h>
#include "foo.h"

void foo(void) {
    printf("shim!\n");
    old_foo();
}

main.c

#include <stdio.h>
#include "foo.h"

int main(int argc, char * argv[]) {
    foo();
    return 0;
}

Makefile

main: main.c shim.o
shim.o: foo.c foo_shim.c foo.h
	#
	# Compile Files
	gcc foo.c      -c -o foo.o
	gcc foo_shim.c -c -o foo_shim.o
	#
	# Rename the symbol we care about: foo -> old_foo
	objcopy --redefine-sym foo=old_foo foo.o _foo.o
	#
	# Link foo_shim, define shim_foo as foo
	ld foo_shim.o -r -defsym shim_foo=foo -o _foo_shim.o
	#
	# Smush it all together
	ld _foo.o _foo_shim.o -r -o shim.o
clean:
	rm -f main
	rm -f *.o

Output:

$ ./main
shim!
foo!