Edit the top level Makefile and set ARCH
Read Kernel/platform-$ARCH/README
Adjust anything it recommends for your platform

The Z80 build tools get stuck in /opt/fcc as will others in future. Right now
/opt/fcc isn't moveable (yes we suck) but you can create that directory and
chown it to the building user.

Make sure you have the needed compilers on your path
If you are running cc65 remember you also need CC65_HOME set properly!

If you are using the lwtools gcc for 6809 ensure that $(pwd)/BUILD/tools is
on your path.

Cross fingers, type "make"

This should give you a build of libraries, user and kernel.
Standalone/filesystem-src contains some scripts to build filesystems
fuzix.org contains a bunch ready to download

For Z80 your VM needs at least 16GB including swap because sdcc is a bit of a
memory monster. Also use sdcc 3.7.1 or later as 3.7.0 is broken in ways that
cause Fuzix to break.

Fuzix expects yacc to be yacc not Bison. if your system for some reason thinks
that executing yacc should run bison you'll have a build failure. For Debian
and derivatives (eg Ubuntu) this can be fixed with

 - sudo apt-get install -y byacc
 - sudo update-alternatives --set yacc /usr/bin/byacc