Fix bugs and improve that about HPX_HAVE_CXX11_AUTO_RETURN_VALUE of CMake.

[taeguk]

This is related to #2720.

1. When uses C++11, HPX_CXX11_AUTO_RETURN_VALUE_DEFAULT must be OFF.
2. The auto return value is C++14 feature. So, renaming to HPX_HAVE_CXX14_AUTO_RETURN_VALUE seems better.

@hkaiser @gentryx Can you review this?

Added later)

3. Add feature test.
4. The formal naming of auto return value is 'return type deduction'. So, renaming to HPX_HAVE_CXX14_RETURN_TYPE_DEDUCTION seems better.
5. Add cmake known feature test.

[hkaiser]

1.When uses C++11, HPX_CXX11_AUTO_RETURN_VALUE_DEFAULT must be OFF.

I think it should be ON for conforming compilers. I agree, this is unusual, but we couldn't come up with a better name. We could however rename the PP constant to HPX_CXX11_NO_AUTO_RETURN_VALUE_DEFAULT and flip the flag.

Rename HPX_HAVE_CXX11_AUTO_RETURN_VALUE to HPX_HAVE_CXX14_AUTO_RETURN_VALUE.

The feature in question is a C++11 feature. C++11 allows for using auto on a function without specifying an explicit return type if the function consists of only a single return statement allowing for the compiler to deduce the return type from this.

[taeguk]

The feature in question is a C++11 feature. C++11 allows for using auto on a function without specifying an explicit return type if the function consists of only a single return statement allowing for the compiler to deduce the return type from this.

@hkaiser Does the feature is a C++11 feature really?? But, I got compile errors related to auto return value with C++11... (https://gist.github.com/taeguk/a9a37590471f0c30a706ca6aea0a392f)
As I think, maybe you confused it with deduced return type for "lambda". (https://isocpp.org/wiki/faq/cpp14-language#generalized-return)

[hkaiser]

@taeguk, you're right - sorry for the confusion on my end. So this is a C++14 facility after all ...

[hkaiser]

LGTM, thanks!

[gentryx]

In this case we might want an overridable configure time test to detect whether this setting should on or off. We had one in 8599ce3

[taeguk]

@gentryx Do you mean adding feature tests for auto return value?

[taeguk]

Wait. What's the problem with CUDA?
Is it insufficient to use just feature test for detecting whether auto return value can be used?

[hkaiser]

Is it insufficient to use just feature test for detecting whether auto return value can be used?

We run the feature tests not for NVCC but for the compiler used by NVCC to generate the code. NVCC itself is just a preprocessor which generates C code from the C++ input.

[gentryx]

@taeguk Yes, that's what I meant.