Configure: pull 'which' back.

At earlier point 'which' was replaced with IPC::Cmd::can_run call.
Unfortunately on RPM-based systems it is a separate package and it's
not given that it's installed. Resurrected 'which' provides
poor-man fallback for IPC::Cmd::can_run.

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