Fix superbuild with OpenCV4

Summary

This merge request fixes Superbuild when OpenCV is used.

Rationale

Compiling the Superbuild with OpenCV leads to the following error :

CMake Error at /usr/share/cmake-3.10/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
  Could NOT find OpenCV (missing: OPENCV_core_LIBRARY) (found version
  "4.1.1")

See issue #2079 (closed) for an example.

In the superbuild the OPENCV_DIR variable is set to ${SB_INSTALL_PREFIX}/share/OpenCV. However this directory does not exist in a OpenCV 4 build.

This line is not needed, as otb is able to find OTB in ${SB_INSTALL_PREFIX} (which is in the CMake search path). The FindOpenCV.cmake script will look for openCV includes in ${SB_INSTALL_PREFIX}/opencv4/opencv2 and the lib are built in ${SB_INSTALL_PREFIX}/lib.

Note that the bug does not appear on CI as the builds are executed in two parts. First the dependencies are built using the OTB_DEPENDS target and then OTB is built independently. The Superbuild is never executed entirely.

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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