Bug#319825: $() isn't bash specific

[Justin Pryzby]

# This might be archived, and so might need a new bug..
reopen 319825
thanks

$() isn't bash specific, and is required by SUS:
  http://www.opengroup.org/onlinepubs/007908799/xcu/chap2.html#tag_001_006_002
(Although it isn't "widely-supported", see info autoconf/portable
shell).  So the thing that enables this shouldn't be "bash", but
something much more general, if not for all #! /bin/sh scripts.