.. I should add that one could do this in a CMakeLists.txt:
include_directories(${Qt5Gui_OPENGL_INCLUDE_DIRS})
Except that Qt5GuiConfigExtras.cmake doesn't set that either (The list of
include-directories to add is empty). That would make it easier / more stand-
outtish to add that include directory to things that #include <QtGui>.
[ade]