Makefile: don't re-define PERL_DEFINES

Since 07d90eadb50 (Makefile: add Perl runtime prefix support, 2018-04-10) we have been declaring PERL_DEFINES right after assigning to it, with the effect that the first PERL_DEFINES was ignored. That bug didn't matter in practice since the first line had all the same variables as the second, so we'd correctly re-generate everything. It just made for confusing reading. Let's remove that first assignment, and while we're at it split these across lines to make them more maintainable. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>