Since this bug still exists in Homebrew as of July 2018, I thought I would
share how I solved it on OS X Sierra v10.12.6

brew edit cask emacs

This will open a text editor. Search for “def install”

Paste the following line immediately below “def install”:

ENV[“cflags"]="-DMAC_OS_X_VERSION_MAX_ALLOWED=101200”

Close the editor and reinstall the cask:

brew reinstall cask emacs