Makefile: don't re-define PERL_DEFINES

Since 07d90ea (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>

Makefile

@@ -2270,9 +2270,10 @@ perl_localedir_SQ = $(localedir_SQ)
 
 ifndef NO_PERL
 PERL_HEADER_TEMPLATE = perl/header_templates/fixed_prefix.template.pl
-PERL_DEFINES = $(PERL_PATH_SQ):$(PERLLIB_EXTRA_SQ):$(perllibdir_SQ)
-
-PERL_DEFINES := $(PERL_PATH_SQ) $(PERLLIB_EXTRA_SQ) $(perllibdir_SQ)
+PERL_DEFINES :=
+PERL_DEFINES += $(PERL_PATH_SQ)
+PERL_DEFINES += $(PERLLIB_EXTRA_SQ)
+PERL_DEFINES += $(perllibdir_SQ)
 PERL_DEFINES += $(RUNTIME_PREFIX)
 
 # Support Perl runtime prefix. In this mode, a different header is installed