Ensure that WakeOnLan runs also on RISC OS 4.x and 6.x

[pzaino]

Background

WakeOnLan works fine on RISC OS 5 and builds using GCC. Next step is to test and fix (where needed) issues to ensure it runs fine also on RISC OS 4.xx and 6.xx

Do not commit directly in the main branch, instead create your branch or fork, make your changes, test them and then ask for a PR to develop branch.

Multiple PRs are ok :)

Definition of Done

• Add fixes to compile with older releases of GCC that runs on RISC OS 4.xx and 6.xx
• Ensure it builds fine on both RISC OS 5 and the older two releases
• Document you changes