Multithreaded compiling is not supported.
I have built the engine in MT mode, and it ran so slow it's not worth bothering with, like 15% the speed the ST mode runs at. BMX's multithread implementation is mostly useless, because it relies on a different garbage collector that is much slower. Making BMX garbage-collected was a huge design mistake, in my opinion, and this is just one more reason why.
The engine also relies on the reference-counted (ST) GC at some points, and will crash without it.