Build Opencv with Qt5 using -DWITH_QT5=ON

...tested (I need to figure out where the sample files are). Includes a new HAVE_QT5 define that is very similar to the original HAVE_QT define except in window_QT.h where it is used to include the qt5 widget headers instead of the qt4 gui headers.

Note: https://qt-project.org/forums/viewthread/23459/ explains why window_QT.cpp line 3103 checks for QT_OPENGL_ES_2. GL_PERSPECTIVE_CORRECTION_HINT is not defined in gl2.h. If someone knows the equivalent call in opengl es2 it should probably be included.