Remove zero length NEWS on mate-netspeed

Signed-off-by: Willy Sudiarto Raharjo <willysr@gmail.com>
Update Build Script
It will check for duplicate sources on every package. This avoids problem when the SlackBuild is updated to point to new version while the old version source remains.

Also added some documentation for readability and maintainability

Thanks to Chess Griffin for reviewing this patch

Signed-off-by: Willy Sudiarto Raharjo <willysr@gmail.com>