Thank you for the reply.
I don't think it was the Avocet compiler or the GCC toolchain. I have only these clues from the source archive:
1. No discernable project files.
2. Some files called DEFAULT.ENV which contain the following:
LIBPATH=c:/arch11/lib
GENPATH=../h;c:\arch11\lib
ERRORFILE=ERRFILE
3. Some GNU-ish makefiles:
CC = chc11
CPP = cpp
ARCH11 = arch11
LINK = linker
ASM = ahc11
Some web sites that I've visited imply that chc11 -> HIWARE -> Archimedes -> Metrowerks -> Freescale, but I can't find any downloads from Freescale for the HC11 family.