title: Qt Creator : Adding an external library (header file not found)
body: <p>This is my external library location in my <code>Qt</code> project (named <code>contr</code>):</p>
  
  <p><a href="https://i.stack.imgur.com/NytEr.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/NytEr.png" alt="enter image description here"></a></p>
  
  <p>This is my <code>makefile</code> :</p>
  
  <pre><code>QT       += core gui
  
  greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
  
  TARGET = contr
  TEMPLATE = app
  
  INCLUDEPATH += 3rdparty/libs/libusb
  
  SOURCES += main.cpp\
          mainwindow.cpp
  
  HEADERS  += mainwindow.h
  
  FORMS    += mainwindow.ui
  
  LIBS += -L"3rdparty/libs/libusb" -llibusb
  </code></pre>
  
  <p>In a file, I have this <code>include</code> :</p>
  
  <pre><code>#include &lt;libusb.h&gt;
  </code></pre>
  
  <p>When I compile, I get the following error :</p>
  
  <pre><code>mainwindow.cpp:3: erreur : C1083: Cannot open include file: 'libusb.h': No such file or directory
  </code></pre>
  
  <p><strong>EDIT</strong> :</p>
  
  <p>I work on <code>Windows</code></p>