Please take in mind that the source release isn't targeted at cross compilation. I am generally building the SDK on MorphOS and test there. bigfoot should be releasing his cross build script soon - that hopefully won't require any modifications to build.
I'm creating a crosstools script myself, and a docker environment for those who'd like to use it as well.
Current version is only gcc-6 + SDK from 2018, so I'm updating it to latest sdk + gcc-9 now.