arg.cpp: Fix building on Visual Studio

Hi,

From the commit message:

Commit 0df7ca27 introduced syntatic changes that required features in C++-17 in Visual Studio to build, in lines 497 and 499, so override the C++ standard on Visual Studio when building libgjs. Since we already require Visual Studio 2017 15.6.x, this should not affect compatibility adversely.

Namely, when building the code with /std:c++14 (the implication of cpp_std=c++14 in the Meson build files) , I get the error C2429 which indicates that I must activate /std:c++17 for the code to build.

With blessings, thank you!