make .qmake.{super,cache,conf} find "anchored" features

these files function as "anchors" for mkspecs/ and features/ directories
used by projects which load these files. ironically, these files didn't
see these feature files themselves.

Change-Id: I590855eb4a9d2c72b9abfcaa431d2f85a719c6e2
(cherry picked from qtbase/7456dfb4552c4e4f900af9fa8923828a23737280)
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/shared/proparser/qmakeevaluator.cpp

@@ -1161,6 +1161,8 @@ bool QMakeEvaluator::loadSpec()
 
     {
         QMakeEvaluator evaluator(m_option, m_parser, m_vfs, m_handler);
+        evaluator.m_sourceRoot = m_sourceRoot;
+        evaluator.m_buildRoot = m_buildRoot;
         if (!m_superfile.isEmpty()) {
             valuesRef(ProKey("_QMAKE_SUPER_CACHE_")) << ProString(m_superfile);
             if (evaluator.evaluateFile(