Thanks, Glenn. This worked. Why isn't this in the INSTALL file?

Eli, you mentioned I needed to set a couple variables. What are these
variables, and what do I need to set them to?

On Tue, Jul 18, 2017 at 5:45 AM, Glenn Morris <rgm@gnu.org> wrote:

>
> >> xml.c:23:10: fatal error: 'libxml/tree.h' file not found
>
> https://debbugs.gnu.org/18779
> https://lists.gnu.org/archive/html/emacs-devel/2015-11/msg01926.html
>
> TL;DR:
> xcode-select --install
>