change implementation of deprecated qtAddLibrary() to modify QT

... instead of invoking qtAddModule() directly.
gives better decoupling and unified code paths.

Change-Id: I4a456ae3b8027aa65b8a4fba7ee5c171ae89be0c
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

mkspecs/features/qt_functions.prf

@@ -38,7 +38,8 @@ defineTest(qtAddLibrary) {
     # Reverse-engineer the module name from the library name.
     for(var, QT_MODULES) {
         isEqual(QT.$${var}.name, $$1) {
-            qtAddModule($$var, LIBS)
+            QT += $$var
+            export(QT)
             return(true)
         }
     }