title: QMake - How to add and use a variable into the .pro file
body: <p>I have a qmake file generated by Qt creator.
  I am modifying it but I do not manage to understand how to create a variable.</p>
  
  <p>For example, I want to declare the library MYPATH as I did here:</p>
  
  <pre><code>MYPATH = /lib/aaa/bbb
  unix:!macx:!symbian: LIBS += -L$(MYPATH)
  </code></pre>
  
  <p>When I run qmake I find in the generated makefile</p>
  
  <pre><code>LIBS = ....... -L$(MYPATH) .....
  </code></pre>
  
  <p>But the MYPATH variable is not declared anywhere.</p>
  
  <p>Does anyone know how to declare such a variable properly?</p>