gpgme 1.8 build failure (again)

[Robert J. Hansen]

Last month I had a build failure for GPGME 1.8 on macOS.  I reported it
to the list and received a useful answer about a custom preprocessor
define that should've been, but was apparently was not, used.

Unfortunately I didn't write this down, and the email thread is not in
the archive.  (Only one message seems to be:
https://lists.gnupg.org/pipermail/gnupg-users/2016-November/057047.html )

Can anyone give me once again the magic invocation needed?  Thanks.  :)