#!/usr/bin/perl

use strict;
use warnings;

use feature 'say';

while (<DATA>) {
  # should you need it, the value is in $2, undef if not present
  say $1 if m{^ \s* \#define \s+ (\S+) (?:\s+ (0|1))? \s* $}x;
}

__DATA__
  #define thread 1
  #define flag
  #define code 0
  #define const (100/5)
  #define flag_condn 1