using ${var:-value} substitutions in config.sub

[Ben Elliston]

Hi John

The Autoconf shell portability guide [1] says this about ${var:-value}
substitutions (which, I should point out, *are* in the POSIX shell):

  ${var:-value}
    Old BSD shells, including the Ultrix sh, don't accept the colon
    for any shell substitution, and complain and die. Similarly for
    ${var:=value}, ${var:?value}, etc.

[1] 
https://www.gnu.org/software/autoconf/manual/autoconf-2.64/html_node/Shell-Substitutions.html#Shell-Substitutions

So, the question is, should we use these in config.sub? My instinct is
that we should allow anything that is in POSIX sh.  I'm sure someone
out there is still running Ultrix, but they shouldn't be. ;-)

@eggert, what do you think?

Cheers, Ben

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