Raise minimum to -std=c++14 and remove support for CUDA 8.0 and gcc 4.9

[BenjaminW3]

C++14 allows us to remove boilerplate code and lets us use new language
features (auto return type deduction, std::integer_sequence, generic
lambdas, ...).
GCC 4.9 as well as CUDA 8.0 do not support C++14 fully and are therefore
not supported anymore.

This removes more than 700 lines of boilerplate code! 🎉

[BenjaminW3]

@ComputationalRadiationPhysics/alpaka-developers I would like to get your feedback:
When C++14 is enabled (-std=c++14) then many of the standard library functions are now constexpr.
This reults in many errors of the kind: error #2891: calling a constexpr __host__ function("operator()") from a __device__ function("foldr") is not allowed. The experimental flag '--expt-relaxed-constexpr' can be used to allow this. because std::plus, std::multiplies, std::min, std::max, std::epsilon and friends are now constexpr. So they can not be used in C++14 within kernels and large parts of alpaka when '--expt-relaxed-constexpr' is not defined.
This problem is independent of this PR and already exists independentently of making C++14 the default. It is not visible in CI because we do not test this combination.
My ask is now: Can we make the definition of --expt-relaxed-constexpr mandatory?

[ax3l]

Good call, has my support. (Many CUDA/C++ developers I know argue this should be default in NVCC anyway.)

[psychocoderHPC]

This problem is independent of this PR and already exists independentently of making C++14 the default. It is not visible in CI because we do not test this combination.
My ask is now: Can we make the definition of --expt-relaxed-constexpr mandatory?

I would like it. The only problem with this flag is that it is pulling all host code signatures into the device compile path. Compiling will fail if you include a host library header e.g. jsoncpp in the user code where bitfields are used for host structures because nvcc is not supporting bitfields on the device.

[ax3l]

Just use nlohmann/json instead ;) https://github.com/nlohmann/json

[BenjaminW3]

@psychocoderHPC

The only problem with this flag is that it is pulling all host code signatures into the device compile path.

Shouldn't it only bring the constexpr __host__ functions into the device compile path? Does it also bring all structs into the device compile path even when they do not have a contexpr constructor?

[psychocoderHPC]

I tried to reproduce the bitfield issue in a mini-app but without success.

The original error was with the lib jsonCpp -> json.h(647): error: "Bitfields and field types containing bitfields are not supported in packed structures and unions for device compilation!"
Since the used app is closed source it is not possible to provide the original code :-(

@BenjaminW3 I think the future development of alpaka without host side constexpr functions will become very hard, so you have green light from my side to make --expt-relaxed-constexpr mandatory.

[psychocoderHPC]

@BenjaminW3 could you please rebase to resolve the merge issue

[BenjaminW3]

I resolved the conflicts.

[BenjaminW3]

I finally got a green build. Please approve and merge! I already have 3 follow-up PRs in the queue ;-)