Modifying Makefile

I'm modifying postgreSQL code and adding a new file (say new.c) under dir
"src/backend/parser". This file calls functions (PSQLexec, PQntuples,
PQgetvalue, ...).

The file looks like:
--------------------------------
#include "libpq-fe.h"
#include "common.h"
bool  exec_sql()
{
  PGresult      *res;
  char          str[1000];
  int           TuplesNums;

  strcpy(str, "SELECT id, fname FROM T;");
  res = PSQLexec(str, false);
  if (!res)
    return false;
  TuplesNums = PQntuples(res);
  PQclear(res);

  return true;
}
-------------------------

I updated the src/backend/parser/Makefile and added "new.o" to the OBJS
list, but when I compile I receive error:
....undefined reference to `PSQLexec'
....undefined reference to `PQntuples'
....undefined reference to `PQclear'

Do you have an idea which Makefile should I update and how? because I'm not
familiar with writing Makefiles.

Thanks in advance