Added backward compatibility to Bash versions prior to version 3.1. The += notation is supported since Bash 3.1, but Jetty is deployed in evironments where it is impossible to upgrade Bash. This commit replace the += notation by a more portable notation.

Also-by:  Louis-Félix Tessier <louisfelix@gmail.com>
Signed-off-by: Greg Wilkins <gregw@intalio.com>
Merge branch 'jetty-9.2.x' into pr/23