Add `/std:c++latest` option to VC `CXX_ABI` build.
MODIFY
target/appveyor.rb
target/appveyor.rb
@@ -33,6 +33,8 @@ MRuby::Build.new('cxx_abi') do |conf|
toolchain :visualcpp
conf.gembox 'full-core'
+ # C compiler settings
+ conf.cc.flags << "/std:c++latest"
conf.compilers.each do |c|
c.defines += %w(MRB_GC_FIXED_ARENA)
end

Add `/std:c++latest` option to VC `CXX_ABI` build.