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.

interface/wx/defs.h

@@ -1545,6 +1545,17 @@ typedef double wxDouble;
 */
 #define wxCHECK_CXX_STD(stdver)
 
+/**
+    Returns @true if the compiler is using the C++17 standard and the header @a header exists.
+    This is designed to guard inclusion of C++17 standard library headers, since MSVC will warn
+    if a header for a newer C++ standard is included when compiling for an older standard.
+
+    @since 3.3.0
+
+    @header{wx/defs.h}
+*/
+#define wxHAS_CXX17_INCLUDE(header)
+
 /**
     This macro can be used in a class declaration to disable the generation of
     default assignment operator.