How to identify platform/compiler from preprocessor macros?

See: http://predef.sourceforge.net/index.php

This project provides a reasonably comprehensive listing of pre-defined #defines for many operating systems, compilers, language and platform standards, and standard libraries.

For Mac OS:

#ifdef __APPLE__

For MingW on Windows:

#ifdef __MINGW32__

For Linux:

#ifdef __linux__

For other Windows compilers, check this thread and this for several other compilers and architectures.