Add new macro for standard library header inclusion

Newer standard library headers should only be included when the compiler
is targetting that standard, otherwise some compilers (like MSVC) will
warn that you are using a newer C++ include on an older version.

include/wx/defs.h

@@ -205,6 +205,15 @@
     #define wxCHECK_CXX_STD(ver) 0
 #endif
 
+/**
+ * C++ header checks
+ */
+#if defined(__has_include)
+    #define wxHAS_CXX17_INCLUDE(header) (wxCHECK_CXX_STD(201703L) && __has_include(header))
+#else
+    #define wxHAS_CXX17_INCLUDE(header) 0
+#endif
+
 /*  ---------------------------------------------------------------------------- */
 /*  check for native bool type and TRUE/FALSE constants */
 /*  ---------------------------------------------------------------------------- */

include/wx/string.h

@@ -46,12 +46,10 @@
 #include <utility>
 
 // Check if C++17 <string_view> is available
-#ifdef __has_include
-    #if __has_include(<string_view>)
-        #include <string_view>
-        #ifdef __cpp_lib_string_view
-            #define wxHAS_STD_STRING_VIEW
-        #endif
+#if wxHAS_CXX17_INCLUDE(<string_view>)
+    #include <string_view>
+    #ifdef __cpp_lib_string_view
+        #define wxHAS_STD_STRING_VIEW
     #endif
 #endif