Implement wxCmpNatural() using the native function under macOS

Use [NSStting localizedStandardCompare:] which is supposed to use the
same order as Finder does.

src/common/arrstr.cpp

@@ -897,11 +897,11 @@ int wxCMPFUNC_CONV wxCmpNaturalGeneric(const wxString& s1, const wxString& s2)
 // ----------------------------------------------------------------------------
 
 // If native natural sort function isn't available, use the generic version.
-#if !defined(__WINDOWS__)
+#if !(defined(__WINDOWS__) || defined(__DARWIN__) || defined(__WXOSX_IPHONE__))
 
 int wxCMPFUNC_CONV wxCmpNatural(const wxString& s1, const wxString& s2)
 {
     return wxCmpNaturalGeneric(s1, s2);
 }
 
-#endif // #if !defined( __WINDOWS__ )
+#endif // not a platform with native implementation

src/osx/cocoa/utils_base.mm

@@ -266,3 +266,10 @@ bool wxCocoaLaunch(const char* const* argv, pid_t &pid)
 }
 
 #endif
+
+int wxCMPFUNC_CONV wxCmpNatural(const wxString& s1, const wxString& s2)
+{
+    // The values of NSOrdered{Ascending,Same,Descending} are the same as
+    // expected return values of wxCmpNatural(), so we don't need to convert.
+    return [wxCFStringRef(s1).AsNSString() localizedStandardCompare: wxCFStringRef(s2).AsNSString()];
+}

tests/arrays/arrays.cpp

@@ -844,3 +844,9 @@ TEST_CASE("wxCmpNaturalGeneric", "[wxString][compare]")
     CHECK(wxCmpNaturalGeneric("a5th 5", "a 10th 10") > 0);
 }
 
+TEST_CASE("wxCmpNatural", "[wxString][compare]")
+{
+    // We can't expect much from the native natural comparison function as it's
+    // locale-dependent, so just run a simple sanity test
+    CHECK(wxCmpNatural("same", "same") == 0);
+}