[gjs/master.msvc] arg.cpp: Fix buildiung on Visual Studio

[Chun-wei Fan <fanchunwei src gnome org>]

commit c902085ed4cd103d458a01c87b9d7027b09f6345
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Fri May 29 16:38:43 2020 +0800

    arg.cpp: Fix buildiung on Visual Studio
    
    Commit 0df7ca2 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.

 meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/meson.build b/meson.build
index 459f96c3..5fc92b83 100644
--- a/meson.build
+++ b/meson.build
@@ -502,6 +502,7 @@ libgjs = shared_library(meson.project_name(),
     link_args: link_args, link_depends: [symbol_map, symbol_list],
     link_with: libgjs_jsapi,
     dependencies: libgjs_dependencies,
+    override_options: cxx.get_id() == 'msvc' ? ['cpp_std=c++17'] : [],
     version: '0.0.0', soversion: '0',
     gnu_symbol_visibility: 'hidden',
     install: true)