Do not build local copies of any of the dependencies on macOS.
(We use the MacPorts versions of them.)
--- lib/Global.cmake.orig	2019-11-21 12:30:13.000000000 -0600
+++ lib/Global.cmake	2019-11-23 14:24:20.000000000 -0600
@@ -9,7 +9,7 @@
 link_directories(${LocalPrefix}/lib)
 
 # Set flags indicating what we should build
-if (APPLE)
+if (FALSE)
   set(BUILD_JSONCPP true)
   set(BUILD_XIPH true)
   set(BUILD_PROTOBUF true)