title: Using qmake variables
body: <p>Is there a way to get qmake to substitute a custom variable like this? In other words, I want $(LIBS) to be replaced with the variable LIBS, so that I can easily change the include path from machine to machine. Many thanks in advance!</p>
  
  <pre><code>LIBS = c:/tmp/libs.pfo
  INCLUDEPATH += . \
  $(LIBS)/OpenCV2.1-msvc/include \
  $(LIBS)/OpenCV2.1-msvc/modules/core/include \
  $(LIBS)/OpenCV2.1-msvc/modules/imgproc/include \
  ...
  </code></pre>
  
  <p>I have also tried this to no avail: </p>
  
  <pre><code>LIBS = c:/tmp/libs.pfo
  INCLUDEPATH += . \
  $$quote($$LIBS/OpenCV2.1-msvc/include) \
  $$quote($$LIBS/OpenCV2.1-msvc/modules/core/include) \
  $$quote($$LIBS/OpenCV2.1-msvc/modules/imgproc/include) \
  ...
  </code></pre>