Since Meson 0.51, there are special build options for "native:true"
builds, prefixed with "build.".  This change breaks cross builds
because `GenParseName.cxx` is no longer built with `-std=c++17`.

This patch adds defaults for "build.c_std" and "build.cpp_std".

Closes https://github.com/MusicPlayerDaemon/MPD/issues/890