Don't set CMAKE_CXX_STANDARD to 11 by default

Use the compiler default C++ dialect if it is C++11 or higher and only
explicitly request C++11 if the compiler can't compile a small test
using C++11 features by default.

This prevents from unnecessarily adding -std=c++11 to the compiler flags
under Unix, even with compilers using e.g. C++17 by default.

build/cmake/options.cmake

@@ -46,8 +46,6 @@ endif()
 # 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()