Fix kafka dependency

[ktf]

No description provided.

[ktf]

@Barthelemy @vascobarroso we also need this, I guess..

[vascobarroso]

@sy-c could you review ?

[sy-c]

I can't grasp the meaning, but I'm sure this is fine

[ktf]

The point is that if libkafka is not available (hence LIBRDKAFKA_REVISION is not set by alibuild) we should not add the option to CMake.

[knopers8]

It seems it could have introduced an issue in QC PR checks for o2-dataflow:

ninja: error: '/local/workspace/DailyBuilds/DailyO2Dataflow-slc8/daily-tags.cZi7Jrsli5/slc8_x86-64/librdkafka/v2.3.0-14/lib/librdkafka++.so', needed by 'stage/lib64/libO2Framework.so', missing and no known rule to make it

Somehow librkafka++ became a dependency of O2Framework, is that intentional?

[sy-c]

libmonitoring now depends explicitely on kafka. So whatever depends on libmonitoring also depends on kafka.

[ktf]

I think it needs to either be marked as a private dependency, or we need to add the find_package also to O2.

[Barthelemy]

So... who is doing what ?
I cannot reproduce this issue on my mac nor on our jenkins under rhel9 and cs8.