Set default language version through variable

[wdhongtw]

Use CMake reserved variable to specify default C++ language version,
so version can be adjust later by each target accordingly.

Recently I noticed that we can not change the C++ language version for targets
through target_compile_features. It turns out that setting language version
with add_definitions is incompatible with target_compile_features, as
the definition flags got propagated to actual command unconditionally.

This PR try to set the language version in a way that CMake awares.

see also:

• https://cmake.org/cmake/help/latest/variable/CMAKE_CXX_STANDARD.html

[wdhongtw]

I also would like to replace other usage of add_definitions with
add_compile_definitions and add_compile_options, but that require we
increase the minimal CMake version form 3.5 to 3.12.

If what's a acceptable, please let me known. Maybe I can enhance these together. :D