Fix SUBDIRS parsing in autotools

Make this example work:

  SUBDIRS =
  SUBDIRS += \
             src
  SUBDIRS += \
             src2

Change-Id: I9beb8e6b22d2a8bf647590f872e8fe0b2c677eba
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/autotoolsprojectmanager/makefileparser.h

@@ -264,6 +264,7 @@ private:
     bool maybeParseCPPFlag(const QString &term);
 
     bool m_success = false;     ///< Return value for MakefileParser::parse().
+    bool m_subDirsEmpty = false;///< States if last subdirs var was empty
 
     std::atomic_bool m_cancel = false;      ///< True, if the parsing should be cancelled.