CppTools: Avoid QT_SUPPORT macro

It is deprecated, and GCC7 warns about RHS expansion to defined.

Change-Id: I161ee9c566ee12d0e443a9054dd0e5cdadc0518e
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/stringtable.cpp

@@ -56,7 +56,7 @@ QString StringTable::insert(const QString &string)
     if (string.isEmpty())
         return string;
 
-#if QT_SUPPORTS(UNSHARABLE_CONTAINERS)
+#ifndef QT_NO_UNSHARABLE_CONTAINERS
     QTC_ASSERT(const_cast<QString&>(string).data_ptr()->ref.isSharable(), return string);
 #endif