Add a configure test for std::make_unique

Seems like this is the only reliable way to determine whether we
have std::make_unique() or need to provide our own version.

Change-Id: I5b162e0a2d2448d7bc519958c6e85a075812135a
Fixes: QTBUG-71448
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/3rdparty/masm/stubs/wtf/Optional.h

@@ -40,6 +40,7 @@
 #pragma once
 
 #include <QtCore/qglobal.h>
+#include <QtQml/private/qtqmlglobal_p.h>
 
 #include <memory>
 #if __cplusplus > 201402L && QT_HAS_INCLUDE(<optional>)
@@ -83,7 +84,7 @@ private:
 
 #endif
 
-#if __cplusplus < 201402L && !defined(__cpp_lib_make_unique) && !defined(Q_CC_MSVC) && !defined(Q_CC_GHS)
+#if __cplusplus < 201402L && !QT_CONFIG(cxx14_make_unique)
 
 namespace std {
     template<typename T, class ...Args>

src/qml/configure.json

@@ -12,7 +12,25 @@
         }
     },
 
+    "tests": {
+        "cxx14_make_unique": {
+            "label": "C++14 make_unique()",
+            "type": "compile",
+            "test": {
+                "include": "memory",
+                "main": [
+                    "std::unique_ptr<int> ptr = std::make_unique<int>();"
+                ]
+            }
+        }
+    },
+
     "features": {
+        "cxx14_make_unique": {
+            "label": "C++14 make_unique",
+            "condition": "tests.cxx14_make_unique",
+            "output": [ "privateFeature" ]
+        },
         "qml-network": {
             "label": "QML network support",
             "purpose": "Provides network transparency.",