ok, here is the content of mmakefile.src to be placed within src_ico subdir, to be able to build this datatype for any aros platform with aros toolchain:
# $Id: mmakefile.src
include $(SRCDIR)/config/aros.cfg
USER_LDFLAGS := -static
FILES := icoclass
#MM workbench-datatypes-ico : includes linklibs
USER_CFLAGS := -DMYDEBUG $(NOWARN_UNUSED_VARIABLE)
%build_module mmake=workbench-datatypes-ico \
modname=ico modtype=datatype \
files=$(FILES)
$(NOWARN_UNUSED_VARIABLE) flag is because of apparently superfluous
struct Library *libBase = (struct Library *)cl->cl_UserData;
in lines 81, 260, 337 and 377 of icoclass.c