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: NRich Salz <rsalz@openssl.org>
Reviewed-by: NAndy Polyakov <appro@openssl.org>
Reviewed-by: NKurt Roeckx <kurt@openssl.org>