need better C++ standard check
As found out in GNOME/vte#110 (closed), meson appears to only check whether the compiler supports the given standard value to the -std=
argument, but not whether the compiler actually implements that standard version. For example, G++ supports the -std=gnu++17
argument since at least version 5, but https://gcc.gnu.org/projects/cxx-status.html#cxx17 shows that it only has full C++17 support since version 7.0.
Edited by Christian Persch