More specifically, please add something like this in @modules/highgui/src/window_QT.cpp@: modules/highgui/src/window_QT.cpp :

<pre><code class="cpp"> @
#ifdef _WIN32
#include <windows.h>
#else
#include <unistd.h>
#endif
</code></pre> #endif@

Otherwise it doesn't compile in newer gcc:
<pre>
.../opencv/modules/highgui/src/window_QT.cpp: In function ‘int cvWaitKey(int)’:
.../opencv/modules/highgui/src/window_QT.cpp:321:28: error: ‘usleep’ was not declared in this scope
</pre>