meson: set the C++ standard to C++11

Matching the CMake as closely as possible, as Meson doesn't have C++11
feature checks like CMAke does.

meson.build

@@ -3,6 +3,7 @@ project('nlohmann_json',
     version : '3.11.3',
     license : 'MIT',
     meson_version : '>= 0.64',
+    default_options: ['cpp_std=c++11'],
 )
 
 nlohmann_json_dep = declare_dependency(