where is #define __linux__

[Aaron Brashears <aaron@lns.com>]

I'm doing some code which is intended to work on linux and sunos. I
was poking through the header files in /usr/include on my debian box
and found a line in g++-3/stl_config.h which specified:

#if defined(__linux__) 

after a quick test, I found out this is true on linux, and not true on
solaris when using gcc. However, grep -r '__linux__' * doesn't reveal
the location where it is in fact #defined.

Anyone have a clue what's going on? While on the topic, is there a
magic preprocessor definition that lets me know if I'm on
sunos/solaris?