to build release version:
cd source/Irrlicht make -f Makefile.amigaos4 NDEBUG=1
This will compile Irrlicht, create a static lib (libIrrlicht.a), and copy it into the subdirectory lib/AmigaOS4. That's all.
to build debug version:
cd source/Irrlicht make -f Makefile.amigaos4
For cross-compilation you may add -j4 at end to speed up compilation process.