[BUG] Rework how we handle feature test macros

[miscco]

Previously, we would define __cpp_lib_meow which interferes with the host standard library.

Worst case we might actually break user code, because we define some macros earlier than the host library would.

To alleviate this and also let a user check what we actually implement we rename our feature test macros to __cccl_lib_meow

We still keep the naming convention of the standard and try to define the standard macros when applicable.

In addition to renaming we also fix some macros that should not be defined and fix some values of others

Fixes #189

[griwes]

I think this approach is sound, but I don't know how I feel about the prefix you went for. I think I'd prefer something more akin to __cuda_cpp or just __cuda, rather than __cccl. (__cuda is a bit tempting, given how we've gone with __cuda_std__ for the main macro proclaiming we're in libcu++ mode.) I don't know if "cccl" actually exists anywhere in the code right now, and I don't know if we want it to start existing.

[miscco]

I think this approach is sound, but I don't know how I feel about the prefix you went for. I think I'd prefer something more akin to __cuda_cpp or just __cuda, rather than __cccl. (__cuda is a bit tempting, given how we've gone with __cuda_std__ for the main macro proclaiming we're in libcu++ mode.) I don't know if "cccl" actually exists anywhere in the code right now, and I don't know if we want it to start existing.

We actually discussed this in the review hour. Originally I had __cuda_lib_meow but that was rejected because it is not really cuda and lacks relation to C++. After a few suggestions we happily settled on __cccl_lib_meow because that clearly signifies our libraries and will potentially allow us to use feature test macros also for cub / thrust

[griwes]

Let me sleep on that, then; I don't know if that last point you made makes me like this prefix less or more.

[miscco]

Let me sleep on that, then; I don't know if that last point you made makes me like this prefix less or more.

I think we would need to bring this up monday in the code review meeting