[CMake] target_compile_features() uses incorrect C++ version

[Kim Walisch]

Hi,

I have realized that my C++ primesieve project (
https://github.com/kimwalisch/primesieve) is
compiled using -std=gnu++11 with GCC 7.3 and CMake 3.10 (on Ubuntu 18.10
x64) even
though the default C++ version used by GCC 7.3 is C++14 as checked below:

$ g++ -dM -E -x c++  /dev/null | grep -F __cplusplus
#define __cplusplus 201402L

Note that I have not set CXX_STANDARD manually in my CMakeLists.txt so I
was expecting
that my project would be compiled either with -std=gnu++14 or without any
C++ version flag.
I spent some time investigating where the -std=gnu++11 flag comes from and
I found that it is
related to the use of target_compile_features():

target_compile_features(primesieve PRIVATE cxx_auto_type)

This code tells the compiler that my program uses features from C++11 and
that the compiler
should enable C++11 if the default C++ version of the compiler is e.g.
C++98. I would
however expect that if the default C++ version of the compiler is > C++11
CMake would not
add -std=gnu++11 to the compiler flags.

For me this is a CMake bug.

Thanks,
Kim Walisch
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