Since many devs use OS X, it would be helpful to add a mention of OS X in Step 2 (Build). It would also be helpful to add that it is okay to ignore build errors/warnings as long as CPython reports building correctly.

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.

@ned.deily Thanks for the feedback.

I changed the text on the QuickStart build step #2 since it was not clear to some sprinters that UNIX included Mac OS X.

I added link in the text to direct to the Build Dependencies section. Thanks for the suggestion.

The updates look good to me, thanks!  Are you able to push this?  If not, I can do it.

@ned.deily Thanks for the review. Please push for me. Thanks :)

New changeset 56b0001c7a29 by Ned Deily in branch 'default':
Issue #23924: Expand the QuickStart section to mention OS X and link
https://hg.python.org/devguide/rev/56b0001c7a29

Done.  Thanks for the patch!