title: LNK2001: Unresolved external symbol when using qwt plot in Qt creator
body: <p>I am subclassing <strong>QwtPlot</strong>. I have the following error:</p>
  
  <p><strong>moc_myplot.obj:-1: error: LNK2001: unresolved external symbol "public: static struct QMetaObject const QwtPlot::staticMetaObject" (?staticMetaObject@QwtPlot@@2UQMetaObject@@B)</strong>
  I have tried to the following things: run qmake, rebuild, clean, delete debug folder, recompile qwt library. That doesn't help. Here is minimal code:
  myplot.h: </p>
  
  <pre><code>#ifndef MYPLOT_H
  #define MYPLOT_H
  #include &lt;QObject&gt;
  #include &lt;qwt_plot.h&gt;
  
  class MyPlot : public QwtPlot
  {
      Q_OBJECT
  public:
      MyPlot();
  };
  
  #endif // MYPLOT_H
  </code></pre>
  
  <p>myplot.cpp:</p>
  
  <pre><code>#include "myplot.h"
  
  MyPlot::MyPlot()
  {
  
  }
  </code></pre>
  
  <p>Here is <strong>.pro</strong> file:</p>
  
  <pre><code>#-------------------------------------------------
  #
  # Project created by QtCreator 2015-06-22T19:33:24
  #
  #-------------------------------------------------
  
  QT       += core gui
  
  greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
  
  TARGET = MyPlot
  TEMPLATE = app
  
  
  SOURCES += main.cpp\
          mainwindow.cpp \
      myplot.cpp
  
  HEADERS  += mainwindow.h \
      myplot.h
  
  win32:CONFIG(release, debug|release): LIBS += -L$$PWD/../../../../../qwt-6.1.2/lib/ -lqwt
  else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/../../../../../qwt-6.1.2/lib/ -lqwtd
  else:unix: LIBS += -L$$PWD/../../../../../qwt-6.1.2/lib/ -lqwt
  
  INCLUDEPATH += $$PWD/../../../../../qwt-6.1.2/include
  DEPENDPATH += $$PWD/../../../../../qwt-6.1.2/include
  </code></pre>
  
  <p>I am using Qt Creator 3.4.1 Based on Qt 5.4.2 (MSVC 2013, 32 bit). Kit: <strong>Desktop Qt 5.4.2 MSVC2013 64bit</strong>. Compiler: <strong>Microsof Visual C++ Compiler 12.0(amd64)</strong>. If I comment <code>Q_OBJECT</code> macro in <code>myplot.h</code> everything is ok. I can use <code>qwt_plot</code> without subclassing, so that <code>this-&gt;setCentralWidget(new QwtPlot());</code> line in <code>mainwindow.cpp</code> is ok.</p>