port contrib/download_prerequisites script to macOS

[Damian Rouson]

All,

The attached patch modifies the contrib/download_prerequisites script to work on macOS. 
The revised script detects the operating system and adjusts the shasum and md5 commands 
to their expected name and arguments on macOS.  The revised script also uses curl if 
wget is not present.  macOS ships with curl but not wget.

Tested on macOS and Lubuntu and Fedora Linux distributions. 

Ok for trunk?

Damian


2017-04-05  Damian Rouson  <damian@sourceryinstitute.org>

        * download_prerequisites (md5_check): New function emulates Linux
        'md5 --check' on macOS.  Modified script for macOS compatibility.

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