perl: Separate compile-time environment from runtime environment

Make it possible to have a separate and different perl command string
for installable scripts than we use when building, with the
environment variable HASHBANGPERL.  Its value default to the same as
the environment PERL if it's defined, otherwise '/usr/bin/env perl'.

Note: this is only relevant for Unix-like environments.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@openssl.org>