Change ADIOS2_Blosc2_PREFER_SHARED behavior

[vicentebolea]

ADIOS2_Blosc2_PREFER_SHARED should not assume there is always a shared c-blosc2 library available.

C-Blosc2 builds allow to control static and shared builds independently, thus this will cause issues with ADIOS2_Blosc2_PREFER_SHARED=ON (default) for purely static builds (prefer shared != require shared).

Originally posted by @ax3l in #3715 (comment)