Makefile: ease dynamic-gettext-poison transition

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 <redacted>
Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>