From the gnu cpp manual…
__GNUC__
__GNUC_MINOR__
__GNUC_PATCHLEVEL__
These macros are defined by all GNU compilers that use the C preprocessor: C, C++, Objective-C and Fortran. Their values are the major version, minor version, and patch level of the compiler, as integer constants. For example, GCC 3.2.1 will define __GNUC__ to 3, __GNUC_MINOR__ to 2, and __GNUC_PATCHLEVEL__ to 1. These macros are also defined if you invoke the preprocessor directly.
__GNUC_PATCHLEVEL__ is new to GCC 3.0; it is also present in the widely-used development snapshots leading up to 3.0 (which identify themselves as GCC 2.96 or 2.97, depending on which snapshot you have).
If all you need to know is whether or not your program is being compiled by GCC, or a non-GCC compiler that claims to accept the GNU C dialects, you can simply test __GNUC__. If you need to write code which depends on a specific version, you must be more careful. Each time the minor version is increased, the patch level is reset to zero; each time the major version is increased (which happens rarely), the minor version and patch level are reset. If you wish to use the predefined macros directly in the conditional, you will need to write it like this:
/* Test for GCC > 3.2.0 */
#if __GNUC__ > 3 || \
(__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \
(__GNUC_MINOR__ == 2 && \
__GNUC_PATCHLEVEL__ > 0)))