Reproducible stdlib option

[alandefreitas]

Although results are more reproducible now with libc++, when MrDocs runs CMake to generate the compilation database, compilers have different defaults for stdlib, causing inconsistencies in CI.

In particular, when MSVC is used to generate compile_commands.json, and then each command is converted to clang commands by the MrDocs database, but MSVC defaults to a much older standard than GCC and Clang for stdlib, which causes most of these inconsistencies.

Thus, if no C++ standard is set in the command line for the compilation command, MrDocs compilation database should make it consistent by setting it to the same level as Clang instead of using MSVC's C++11.