build: configure should check for bash 4.x or newer (for 'declare -A' support)

[salewski]

This issue pertains to screen-wrappers-0.3.1 (the latest at the time of
writing).

Associative arrays (created with 'declare -A') were added in bash-4.0, and
programs such as 'screen-ls' that use them will bomb out with an error
status if invoked using some earlier version of bash:

    $ screen-ls
    /path/to/bin/screen-ls: line 257: declare: -A: invalid option
    declare: usage: declare [-afFirtx] [-p] [name[=value] ...]

The screen-wrappers 'configure' script should check not only for the
presence of bash (as it does currently), but also for a recent enough version
(major version 4 or newer, as reported in 'BASH_VERSINFO'). Alternatively,
there might be a way to perform a feature test so that any
"bash-looking-thing" could be used, but that might be more trouble than it is
worth.