Makefile: make PERL_DEFINES recursively expanded

Since 07d90eadb50 (Makefile: add Perl runtime prefix support, 2018-04-10) PERL_DEFINES has been a simply-expanded variable, let's make it recursively expanded instead. This change doesn't matter for the correctness of the logic. Whether we used simply-expanded or recursively expanded didn't change what we wrote out in GIT-PERL-DEFINES, but being consistent with other rules makes this easier to understand. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>