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/init.cmake

@@ -10,6 +10,25 @@
 
 if(DEFINED wxBUILD_CXX_STANDARD AND NOT wxBUILD_CXX_STANDARD STREQUAL COMPILER_DEFAULT)
     set(CMAKE_CXX_STANDARD ${wxBUILD_CXX_STANDARD})
+    set(CMAKE_CXX_STANDARD_REQUIRED ON)
+else()
+    # If the standard is not set explicitly, check whether we can use C++11
+    # without any special options.
+    include(CheckCXXSourceCompiles)
+    check_cxx_source_compiles("
+        #include <vector>
+        int main() {
+            std::vector<int> v{1,2,3};
+            for (auto& n : v)
+                --n;
+            return v[0];
+        }"
+        wxHAVE_CXX11)
+    if(NOT wxHAVE_CXX11)
+        # If not, request it explicitly and let CMake check if it's supported.
+        set(CMAKE_CXX_STANDARD 11)
+        set(CMAKE_CXX_STANDARD_REQUIRED ON)
+    endif()
 endif()
 
 if(MSVC)

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()