Simplify accelerator names in CMake

[bernhardmgruber]

To enable/disable accelerators in CMake, there are various options, like:

ALPAKA_ACC_CPU_BT_OMP4_ENABLE
ALPAKA_ACC_CPU_B_OMP2_T_SEQ_ENABLE
ALPAKA_ACC_GPU_CUDA_ENABLE

These names are sometimes confusing (like why is there a B and a T in the name). Furthermore, they do not correspond to the names we use in code. E.g. ALPAKA_ACC_CPU_B_SEQ_T_SEQ_ENABLE corresponds to alpaka::acc::AccCpuSerial in code.

I would like to propose to rename the options in CMake to match the accelerator names in code:

ALPAKA_ENABLE_AccCpuSerial
ALPAKA_ENABLE_AccCpuOmp2Blocks
ALPAKA_ENABLE_AccGpuCudaRt

[bernhardmgruber]

I was about to open an issue about this now and then thought that maybe someone already proposed this. Well, it was me :)

I would like us to revisit this again. It's just to hard (for me) to remember how to spell all the cmake variables on the command line.

We may even go shorter:

alpaka_ENABLE_CUDA
alpaka_ENABLE_SERIAL
alpaka_ENABLE_OMP2_BLOCK
alpaka_ENABLE_OMP2_THREADS

etc.

[bernhardmgruber]

I would like to discuss about the fate of this issue before we release alpaka 1.0, since it breaks a user's cmake use.

[bernhardmgruber]

We discussed that this should go together with #1019 sometime in the future.