Configure: impose ^X on whole build procedure.

Traditionally Configure passed $ENV{PERL} to Makefile. But this
resulted in ambiguilty as Configure script could be executed by
interpreter different from one executing remaining scripts. Since
we separate compile- and run-time interpreters with HASHBANGPERL
variable, there is no reason to segment the build procedure.

Reviewed-by: Rich Salz <rsalz@openssl.org>