In general, references in the devguide to UNIX are intended to include OS X.  In other words, in the guide "UNIX" is generally equivalent to "not Windows" at least for Python 3 now that other, ancient non-POSIX platforms, like classic MacOS or VMS or OS/2 are no longer supported. (At least, I can't think of any remaining ones.) Perhaps adding a statement to that effect somewhere in the guide would help if it isn't already clear?  I think the main difference is whether a platform uses the autoconf-based build system for Python (e.g. ./configure and make) vs. the Windows Visual Studio project build system).

Regarding build errors/warnings, the issue of standard library extensions depending on installing third-party libraries that may not be included by default or at all on a particular OS platform is talked about in various places in the guide (for example, in the Build Dependencies section: https://docs.python.org/devguide/setup.html#build-dependencies).  Perhaps a link to that section can be added to the quick start.