CMake: Add variable to indicate if c++11 or later is enabled

Use it in macOS checks.

build/cmake/setup.cmake

@@ -23,11 +23,7 @@ include(CheckTypeSize)
 include(CMakePushCheckState)
 include(TestBigEndian)
 
-if(
-    APPLE AND
-    CMAKE_OSX_DEPLOYMENT_TARGET VERSION_LESS 10.9 AND
-    (CMAKE_CXX_STANDARD EQUAL 11 OR CMAKE_CXX_STANDARD EQUAL 14)
-  )
+if(APPLE AND CMAKE_OSX_DEPLOYMENT_TARGET VERSION_LESS 10.9 AND wxHAS_CXX11)
     set(CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS} "-stdlib=libc++")
 endif()