update how c++11 requirement is added

[kejxu]

update how c++11 compiler flag is added:

• only add it when it's supported (tested on Ubuntu 18.04)
• add similar logic for MSVC (as mentioned in the comment, this check would fail, but c++11 is always enabled). this pattern could be helpful if later this project moves to c++14 (which is an available switch in MSVC)

the other way to do this is to use target_compile_features(mylib PUBLIC cxx_std_11) from cmake, but this would mean to add this line for every target. would this be preferred instead of adding a global c++11 flag?

[tfoote]

If MSVC is using c++14 by default we shouldn't need to downgrade it for this. Can we just skip the declaration on Windows?

[kejxu]

If MSVC is using c++14 by default we shouldn't need to downgrade it for this. Can we just skip the declaration on Windows?

sure, this was mentioned during our porting too. would we want to keep check_cxx_compiler_flag(-std=c++11 COMPILER_SUPPORTS_CXX11) for G++ or change to just skip this check for MSVC?

[tfoote]

We might as well improve the linux logic to check for it before applying it. And then just skip it on MSVC.

[kejxu]

We might as well improve the linux logic to check for it before applying it. And then just skip it on MSVC.

updated to skip /std:c++11 check for msvc and updated comment