Prevent errors on macOS

Enabled CLBlastFactory::isAvailable() on macOS.
This allows it to be incorporated into initialization processes on macOS that don't actually use OpenCL.
In environments like GitHub's hosted runners where there are no OpenCL devices, CLBlastFactory::isAvailable() can now return false even on macOS.

However, we still haven't performed any actual OpenCL tests on macOS.
This module does not officially support macOS.