Darwin bulk build machines: clang too old

[Thomas Klausner <wiz%netbsd.org@localhost>]

Hi!

After noticing that mame doesn't build in the Darwin bulk builds, I've
filed an upstream bug report and got told that the clang used is too
old.

https://github.com/mamedev/mame/issues/15850

We don't have CLANG_REQD, so I'm not quite sure what to do with this
information, but perhaps the bulk build machine can be upgraded to a
newer clang?
 Thomas