much higher memory usage on "compatible" builds

[JulianGro]

The build VM I have been using for quite some time now has been running out of memory on the RC build.
Lowering to 7 threads I have seen a 14GiB peak in (total system) memory usage inside the VM. Since it's very hard to realize builds failing because they ran out of memory, we should probably apply new memory rules for the thread count when using the "compatible" optimization profile.

[daleglass]

I'll probably add a memory usage estimate for optimized builds