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

Use it in macOS checks.

build/cmake/init.cmake

@@ -11,6 +11,11 @@
 if(DEFINED wxBUILD_CXX_STANDARD AND NOT wxBUILD_CXX_STANDARD STREQUAL COMPILER_DEFAULT)
     set(CMAKE_CXX_STANDARD ${wxBUILD_CXX_STANDARD})
 endif()
+if(NOT CMAKE_CXX_STANDARD EQUAL 98)
+    set(wxHAS_CXX11 TRUE)
+else()
+    set(wxHAS_CXX11 FALSE)
+endif()
 
 if(MSVC)
     # Determine MSVC runtime library flag