Annd it’s back! MAME for macOS now compiles under Mojave and the SDL2 issues have been fixed with the release of SDL2.0.9, make sure you update!
MAME for Raspberry Pi is also compiling again
A macOS build is now available on my MAME page, it was built using the latest Mojave build and XCode 10.1. There is also a Raspberry Pi version built with Raspbian Stretch, a custom SDL2.0.9 and GCC 6.3.