Here's how Makefile.rules was modified to add an additional ECJ check:

# When ECJ is available, use it as an extra lint pass.  GCJ has the
# habit of silently compiling invalid Java code (garbage in, garbage out),
# fortunatly ECJ rejects it.
if HAVE_ECJ
all-local: ecj/checked
endif
ecj/checked: files.list
        rm -rf ecj
        mkdir -p ecj
        $(ECJ) \
                -d ecj \
                -warn:+semicolon \
                -sourcepath $(SOURCEPATH) \
                -classpath $(CLASSPATH) \
                @$(top_builddir)/files.list \
                2>&1 | tee $@.log
        if test -s $@.log ; \
        then rm $@.log ; false ; \
        fi
        rm -f $@.log
        touch $@
.PHONY: clean-ecj-dir
clean-local: clean-ecj-dir
clean-ecj-dir:
        rm -rf ecj

of course for glade files, this would be easier.  No need to worry about
HAVE_ECJ; instead a simplier:
glade.checked:
          for f in x $(glade_DATA) ; do \
                   test "$$f" = x && continue ; \
          ...
would work

This has been added to Makefile.rules in common/ and committed