auto-pkgsrc-setup enhancement

[Jason Bacon <bacon4000%gmail.com@localhost>]

FYI:

The newest auto-pkgsrc-setup allows Linux users to select a minimum GCC 
compiler from among 4.8, 4.9, 5.0, 6.0, and 7.0.

At this time I recommend 4.x or 5.0, as there are hundreds of additional 
failures with gcc6 as of my last pbulk build at the end of October.

Sample pbulk success rates:

RHEL6-gcc6    15849

RHEL6-gcc48   16461

RHEL7-gcc5    16338

RHEL7-gcc48 (Yum)  16414

Either 4.x or 5.0 produces thousands more successful builds than GCC 
4.4.7 on CentOS 6 (which will still be around for a couple more years).

The latest script will always be available here:

https://netbsd.org/~bacon/

Happy Holidays...

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