Honestly curious, since I don’t yet have a Mac this recent, have you tried disabling the Secure Boot feature from the Recovery HD? Will Haiku boot on it then?
How is this relevant?
So in answering why its relevant, Macs can have a hard time booting different editions of Haiku (such as the legacy 32-bit version on newer hardware), so it is useful to learn why. Far as why the hardware itself is relevant, it’s because there’s several members here including myself that love the Mac as much as some here like ThinkPads, etc. and BeOS used to run on Macintosh hardware back then… which ‘ties in’ the past with today/Haiku. The reason Be had to stop supporting Mac hardware was a lack of knowledge of G3 and G4 processors, and so BeOS on the Mac came to a halt. Without understanding how Mac hardware works with Haiku and keeping up to date on it with more recent models (though I confess all the Macs I use are before 2012 at the moment), Haiku could fall to the same fate with Macs today. In a way, I’m hoping Haiku will be able to keep up with running on Apple hardware. Hopefully…
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See: Pre-requisite Software | Haiku Project
“The tools you need to compile software for Haiku, or to compile Haiku itself, depend on the platform that is used for building.”
In macOS, devs needs specific compiler flags to support unsupported cross-platform compilations… 
FYI: we can see the change history of your post with clicking on the orange pencil.
One possibility could be to use another compiler, for example the gcc shipped by brew. All the various packaging systems on Mac: homebrew, fink, etc… need sometimes their own compiler to properly build packages. Sometimes in the past I was able to crosscompile to Windows using MXE, some willing soul could explore the possibility to adapt their setup to cross compile to Haiku, instead of Windows.