[CMAKE]: When threading is disabled, don't attempt to find threads

[ad3154]

Describe your changes

This PR:

• Adds a clear option in CMake for disabling threading
• Stops searching for threads from the root if threading is disabled

How has this been tested?

• Tested compilation with CAN_STACK_DISABLE_THREADS of just isobus.lib doesn't link pthreads, and makes no "Threads found" output

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

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