CMake interaction with C/C++ std flags

[henryiii]

This started in the discussion for #1089. The core issue was that C++ flags mix into C compilation, because PyBind has it's own implementation of C++ flags, and it doesn't include a way to separate C code.

CMake 2.8 was released in 2009, long before C++11 (much less the other standards) were released. Modern CMake does have a way to select C++ versions, and PyBind can be in contention with them.

Method 1: CMake 3.1+: CMAKE_CXX_STANDARD (and CMAKE_CXX_STANDARD_REQUIRED, CMAKE_CXX_EXTENSIONS). The current implementation does include checks for these, and turns off custom flags.
Method 2: CMake 3.1+: Manually listing features with target_compile_features. This was supposed to be the "best" way to do things, but listing every feature in C++11 is tedious.
Method 3: CMake 3.8: Meta-compile features were added for C++ standards, finally allowing target_compile_features(my_target PUBLIC cxx_std_11) or similar.

The first method is checked and appropriately handled by pybind11. The final method is not well supported by pybind11. I believe that by leaving PYBIND_CPP_STANDARD on CMake 3.8+, PyBind should default to this method. This should allow overriding the standard (automatically handled by CMake if you request it on a pybind target), as well as keep the C++/C separate without non-portable generator expressions.

And we can avoid the awful CPP/CXX mixed name by phasing out PYBIND_CPP_STANDARD on new targets 😉.