It's more likely that the compiler will set something that you can use. All (or most) compilers will implicitly declare macros that you can check for in conditional compilations. Look in the help for your particular compiler (probably under "pre-defined macros" or similar).
Correct is better than fast. Simple is better than complex. Clear is better than cute. Safe is better than insecure.
-- Sutter and Alexandrescu, C++ Coding Standards
Programs must be written for people to read, and only incidentally for machines to execute.
-- Harold Abelson and Gerald Jay Sussman
The cheapest, fastest and most reliable components of a computer system are those that aren't there.
-- Gordon Bell