> But I am a bit worried at the new makefile syntax [ifeq directive and $(findstring) function]. I suspect it is Gnu specific, and that Python tries to support other versions of Make as well.

Yes, they are both GNU extensions :(

To avoid modifying configure.ac since the information is already available, the recipes could be modified with the following conditional:

    cross_compiling=$$(echo $(PYTHON_FOR_BUILD) | sed "s/.*_PYTHON_HOST_PLATFORM.*/yes/"); \
    if [ "$$cross_compiling" = "yes" ]; then \
          touch $@;
    else \
          # The original recipes.
    fi

So the built binaries $(PGEN) and Programs/_freeze_importlib would be dummy empty files, but that would not prevent Programs/_freeze_importlib.o, $(LIBRARY_OBJS_OMIT_FROZEN) and $(PGENOBJS) to be needlessly cross-compiled.