Makefile: make PERL_DEFINES recursively expanded

Since 07d90eadb5 (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>

Makefile

@@ -2271,7 +2271,7 @@ perl_localedir_SQ = $(localedir_SQ)
 
 ifndef NO_PERL
 PERL_HEADER_TEMPLATE = perl/header_templates/fixed_prefix.template.pl
-PERL_DEFINES :=
+PERL_DEFINES =
 PERL_DEFINES += $(PERL_PATH_SQ)
 PERL_DEFINES += $(PERLLIB_EXTRA_SQ)
 PERL_DEFINES += $(perllibdir_SQ)