Unix Makefile: Make sure to use $(PERL) when running ./Configure

For consistency, it's better to use the perl that was specified to
Configure last time it was called.

Use case:

perl v5.8.8 was first along $PATH, perl v5.22.2 was available and
specified as: PERL=/opt/local/bin/perl ./config.  When make wanted to
reconfigure and called './Configure reconf', configuration broke down,
complaining about a perl that's too old.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1884)

Configurations/unix-Makefile.tmpl

@@ -769,7 +769,7 @@ tar:
 	cd $(SRCDIR); ls -l $(TARFILE).gz
 
 dist:
-	@$(MAKE) PREPARE_CMD='./Configure dist' tar
+	@$(MAKE) PREPARE_CMD='$(PERL) ./Configure dist' tar
 
 # Helper targets #####################################################
 
@@ -828,7 +828,7 @@ openssl.pc:
 configdata.pm: $(SRCDIR)/Configure $(SRCDIR)/config {- join(" ", @{$config{build_file_templates}}, @{$config{build_infos}}, @{$config{conf_files}}) -}
 	@echo "Detected changed: $?"
 	@echo "Reconfiguring..."
-	$(SRCDIR)/Configure reconf
+	$(PERL) $(SRCDIR)/Configure reconf
 	@echo "**************************************************"
 	@echo "***                                            ***"
 	@echo "***   Please run the same make command again   ***"