find features/ under mkspecs/ of source & build root

we were finding features/ directly in the source & build root, and we
were finding features/ under mkspecs/ from $QMAKEPATH and other mkspecs
locations, but we omitted the "transitive hull". this was
counterintuitive.

Change-Id: I9823e6606467c98f264c81385250da92311f51ca
(cherry picked from qtbase/0bf65180b8710e4516496d0bc53a265ff0dd6a01)
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/shared/proparser/qmakeevaluator.cpp

@@ -1437,10 +1437,14 @@ void QMakeEvaluator::updateFeaturePaths()
             m_option->dirlist_sep, QString::SkipEmptyParts);
 
     QStringList feature_bases;
-    if (!m_buildRoot.isEmpty())
+    if (!m_buildRoot.isEmpty()) {
+        feature_bases << m_buildRoot + mkspecs_concat;
         feature_bases << m_buildRoot;
-    if (!m_sourceRoot.isEmpty())
+    }
+    if (!m_sourceRoot.isEmpty()) {
+        feature_bases << m_sourceRoot + mkspecs_concat;
         feature_bases << m_sourceRoot;
+    }
 
     foreach (const QString &item, m_option->getPathListEnv(QLatin1String("QMAKEPATH")))
         feature_bases << (item + mkspecs_concat);