Prefix VOLK CMake options with `VOLK_*` to avoid global namespace pollution

[xerpi]

When integrating VOLK via FetchContent or add_subdirectory, its CMake options are exposed in the global CMake option namespace. This can lead to unintended conflicts or ambiguity, especially in larger projects where generic option names may overlap.

For example, options like:

ENABLE_STATIC_LIBS
ENABLE_TESTING

are too generic and may clash with similarly named options from other dependencies or the parent project.

All VOLK-specific CMake options should be prefixed with VOLK_ to ensure proper namespacing and avoid polluting the global option space.

Example:

ENABLE_STATIC_LIBS -> VOLK_ENABLE_STATIC_LIBS
ENABLE_TESTING -> VOLK_ENABLE_TESTING

[jdemel]

I agree that this should be improved.
My concern would be: How do we not break the current behavior? Or do we need to do a new major release?

I guess we'd need to do a "VOLK_POLICY_0001" thing and default to "OLD". At least something like that.