Allow the use of IP-clash detection, also when auto-IP or DHCP are not enabled

[htibosch]

Description

See the forum post: "FreeRTOS+TCP Impossible to use ipconfigARP_USE_CLASH_DETECTION without ipconfigDHCP_FALL_BACK_AUTO_IP".

Allow the use of IP-clash detection, also when fall-back to auto-IP is not enabled.
But issue a warning to make the user double-check this setting.

Test Steps

1. Disable ipconfigUSE_DHCP or ipconfigDHCP_FALL_BACK_AUTO_IP
2. Enable ipconfigARP_USE_CLASH_DETECTION

Compile your project, and you will get a compilation error:

#error ipconfigARP_USE_CLASH_DETECTION is unused when ipconfigDHCP_FALL_BACK_AUTO_IP is disabled

This PR will change the #error into a #warning.

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[htibosch]

Can anyone please start the CI-checks for this PR?
Thanks, Hein