Makefile: ease dynamic-gettext-poison transition

Earlier we made the entire build to fail when GETTEXT_POISON=Yes is given to make, to notify those who did not notice that text poisoning is now a runtime behaviour. It turns out that this is too irritating for those who need to build and test different versions of Git that cross the boundary between history with and without this topic to switch between two environment variables. Demote the error to a warning, so that you can say something like make GETTEXT_POISON=Yes GIT_TEST_GETTEXT_POISON=Yes test during the transition period, without having to worry about whether exact version you are testing has or does not have this topic. Signed-off-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>