Guard definitions for provisional extensions with a dedicated opt-in macro

[kpet]

Many new extensions are being developed as provisional extensions. As part of their development the OpenCL WG may need to break backwards compatibility which may sometimes (has in the past) exposed applications that do not need provisional definitions to breaking changes. As discussed in the 2024/03/19, we should guard definitions for provisional extensions with an opt-in macro to limit exposure to breaking changes to those applications that use provisional extensions.