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: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1884)