title: Qt compiler cannot find the header file
body: <p>I know it's a quite common issue but I haven't found a comprehensive answer on the following question.
  I have Qt 5.4.1 MSVC2013 build running on Windows 8.1.
  Here is a look on my project files:</p>
  
  <p><img src="https://i.stack.imgur.com/lcVkx.jpg" alt="enter image description here"></p>
  
  <p>And here is what my .pro file looks like:</p>
  
  <pre><code>QT       += core gui
  
  greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
  
  TARGET = Vfp
  TEMPLATE = app
  
  QMAKE_CXXFLAGS = - std=c++11
  
  SOURCES += main.cpp\
          Views/mainview.cpp
  
  HEADERS  += Views/mainview.h
  
  FORMS    += Views/mainview.ui
  </code></pre>
  
  <p>The problem is, unless I add <code>INCLUDEPATH += Views/</code> to the .pro file, I cannot include <code>mainview.h</code> in <code>main.cpp</code> file.</p>
  
  <p>Why? Shouldn't <code>HEADERS  += Views/mainview.h</code> be enough?</p>