Use C++11 by default in CMake build under macOS

Force the use of C++11 standard on macOS to avoid C++11 specific
warnings with clang.

Closes https://github.com/wxWidgets/wxWidgets/pull/2374

build/cmake/options.cmake

@@ -47,6 +47,8 @@ if(NOT MSVC OR MSVC_VERSION GREATER 1800)
     # support setting the C++ standard, present it an option to the user
     if(DEFINED CMAKE_CXX_STANDARD)
         set(wxCXX_STANDARD_DEFAULT ${CMAKE_CXX_STANDARD})
+    elseif(APPLE)
+        set(wxCXX_STANDARD_DEFAULT 11)
     else()
         set(wxCXX_STANDARD_DEFAULT COMPILER_DEFAULT)
     endif()