Add a directry spec for mcr if there is none

On VMS, the command MCR will assume SYS$SYSTEM: when the first
argument lacks a directory spec.  So for programs in the current
directory, we add [] to tell MCR it is in the current directory.
It's the same as having ./ at the start of a program on Unix so the
shell doesn't start looking along $PATH.

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