Create valid (if empty) "C" locale under macOS

Use [NSLocale systemLocale] instead of trying to create a locale with
the name "C" which just fails and doesn't create any NSLocale at all.

If nothing else, this makes the behaviour of wxUILocale::GetCurrent()
consistent on all platforms as its GetLocaleId() now returns "C"
everywhere.

src/osx/core/uilocale.mm

@@ -131,7 +131,13 @@ wxUILocaleImplCF::Use()
 wxString
 wxUILocaleImplCF::GetName() const
 {
-    return wxCFStringRef::AsString([m_nsloc localeIdentifier]);
+    wxString name = wxCFStringRef::AsString([m_nsloc localeIdentifier]);
+
+    // Check for the special case of the "empty" system locale, see CreateStdC()
+    if ( name.empty() )
+        name = "C";
+
+    return name;
 }
 
 wxLocaleIdent
@@ -209,7 +215,12 @@ wxUILocaleImplCF::GetLayoutDirection() const
 /* static */
 wxUILocaleImpl* wxUILocaleImpl::CreateStdC()
 {
-    return wxUILocaleImplCF::Create(wxLocaleIdent().Language("C"));
+    // This is an "empty" locale, but it seems to correspond rather well to the
+    // "C" locale under POSIX systems and using localeWithLocaleIdentifier:@"C"
+    // wouldn't be much better as we'd still need a hack for it in GetName()
+    // because the locale names are always converted to lower case, while we
+    // really want to return "C" rather than "c" as the name of this one.
+    return new wxUILocaleImplCF([NSLocale systemLocale]);
 }
 
 /* static */