pkg: display output of shell commands on real time if verbose. * pkg/private/shell.m: display output of commands in real time and return empty string when verbose. Also small refactor: create the 'special' command for windows on its condition block rather than calling system() there to reduce code duplication. * pkg/private/configure_make.m: adjust call to shell() in order to display output of `configure' and `make' on real time if verbose.

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