https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69473
--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> --- The purpose of macros like __linux__ or __linux is to enable users to write code that's portable to different versions or distributions of the same OS without necessarily testing it on them all, or even having access to them. The problem with relying on the output of the compiler (or cpp) is that, as bug 28314 points out, it sometimes differs between different compiler options or even target architectures. Since GCC encodes the conditions under which the macros are defined it needs to document them. This could be done in a way that's similar to the Host/Target specific installation notes for GCC (https://gcc.gnu.org/install/specific.html), except with one entry per system.
