title: gtk/gtk.h: No such file or directory
body: <p>In <code>opencv</code> compilation, we need <code>gtk</code>. I have gtk-2.0 and gtk-3.0 installed.
  This command gave me</p>
  
  <pre><code> dpkg -l libgtk2.0-0 libgtk-3-0
  
  Desired=Unknown/Install/Remove/Purge/Hold
  | Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
  |/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
  ||/ Name           Version      Architecture Description
  +++-==============-============-============-=================================
  ii  libgtk-3-0:amd 3.10.8-0ubun amd64        GTK+ graphical user interface lib
  ii  libgtk2.0-0:am 2.24.23-0ubu amd64        GTK+ graphical user interface lib
  ii  libgtk2.0-0:i3 2.24.23-0ubu i386         GTK+ graphical user interface lib
  </code></pre>
  
  <p>I have gtk-2.0 and gtk-3.0 folder inside /usr/include/.
  They exist as /usr/include/gtk-2.0 and /usr/include/gtk-3.0</p>
  
  <p>But my compilation for opencv gave me error as </p>
  
  <pre><code>src/window_gtk.cpp:48:21: fatal error: gtk/gtk.h: No such file or directory
   #include "gtk/gtk.h"
                       ^
  compilation terminated.
  </code></pre>
  
  <p>I have gtk/gtk.h inside both gtk-2.0 and gtk-3.0 folders.
  What could be wrong?</p>