What C preprocessor conditional should I use for OS X specific code?

Question

What C preprocessor conditional should I use for OS X specific code? I need to include a specific library if I am compiling for OS X or a different header if I am compiling for

Answer 1

This list of operating system macros says the presence of both __APPLE__ and __MACH__ indicate OSX.

Also confirmed at line 18 of part of the source for fdisk.