As mentioned in the thread containing this message:

http://lists.gnu.org/archive/html/emacs-devel/2017-05/msg00521.html

the DARWIN_OS_CASE_SENSITIVE_FIXME code was getting in the way of maintenance, 
and doesn't seem to be working or needed or used. I attempted to get it to work 
in the first attached patch (untested), and then removed it as described in the 
second attached patch. If that code ever turns out to be useful someone can 
revert the second patch and give my attempt a whirl.

As I understand it, the original bug was fixed last year; this email is merely 
about trying to make the fix easier to maintain.