Makefile: don't re-define PERL_DEFINES

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>