#include <iostream> 
#include <cstdio>
using namespace std;

#define smth

int main() {

    #ifdef smth
        puts("smth is defined");
    #else 
        puts("smth is not defined");
    #endif

    #ifndef smth // #ifnfed = !ifdef
        puts("smth is not defined");
    #else 
        puts("smth is defined");
    #endif
    return 0;
}

Вот здесь: http://www.codeblocks.org/docs/main_codeblocks_en3.html#x3-270001.11.6

А вообще можно писать

#ifndef ONLINE_JUDGE
  // Local stuff
#endif

так как многими серверами это поддерживается.

This can be done in the compiler settings of your IDE. Usually they allow you to pass some arguments to the compiler invocation. You need to pass the argument:

-D something

You could enable by putting #define something 1 before that #ifdef :

#define something 
#ifdef something
...
#endif

you could enable by putting #define something 1 before that #ifdef :

#define something
#ifdef something
...
#endif

This is a simple example for #ifdef, #ifndef and #else you can also have #elif which works like else if and there is #if that works like if;

#define HelloWorld

#ifdef HelloWorld
#define ByeWorld
// This line will be executed
#else
#define HelloWorld
// This line won't be executed
#endif

#ifndef HolaMundo
#define HolaMundo
// This line will be executed
#else
#define AdiosMundo
// This line won't be executed
#endif

"something" можно передать в аргументах компилятора:

g++ -Dsomething test.cpp

Если ваш проект в CodeBlocks использует CMake, откройте файл CMakeLists.txt и добавьте туда строчку

add_definitions(-Dsomething)

там же можно добавить все опции, перечисленные в этом посте, например:

add_compile_options("-std=c++11" "-O2" "-Wall" "-Wextra")

For Codeblocks: Settings -> Compiler.

If you invoke gcc manually: g++ -DSOMETHING ...