Handle absolute OPENCV_INCLUDE_INSTALL_PATH correctly

In case OPENCV_INCLUDE_INSTALL_PATH is absolute (i.e. starts with a "/"),
the path ends up with a double "/".

While this is mostly equivalent to a single slash, it may have a nasty
side effect when:
- OpenCV_INSTALL_PATH is empty
- OPENCV_INCLUDE_INSTALL_PATH is "/usr/include"
- the calling build script uses "-isystem" to specify the path to the
  headers of dependencies (to avoid warnings)

Specifying "-isystem /usr/include" breaks the path ordering, and GCC can
no longer find its "stdlib.h", thus CMake filters such statements.
Unfortunately it fails to do so when using "//usr/include".

cmake/OpenCVGenConfig.cmake

@@ -68,7 +68,11 @@ configure_file("${OpenCV_SOURCE_DIR}/cmake/templates/OpenCVConfig-version.cmake.
 #  Part 2/3: ${BIN_DIR}/unix-install/OpenCVConfig.cmake -> For use *with* "make install"
 # -------------------------------------------------------------------------------------------
 file(RELATIVE_PATH OpenCV_INSTALL_PATH_RELATIVE_CONFIGCMAKE "${CMAKE_INSTALL_PREFIX}/${OPENCV_CONFIG_INSTALL_PATH}/" ${CMAKE_INSTALL_PREFIX})
-set(OpenCV_INCLUDE_DIRS_CONFIGCMAKE "\"\${OpenCV_INSTALL_PATH}/${OPENCV_INCLUDE_INSTALL_PATH}\" \"\${OpenCV_INSTALL_PATH}/${OPENCV_INCLUDE_INSTALL_PATH}/opencv\"")
+if (IS_ABSOLUTE ${OPENCV_INCLUDE_INSTALL_PATH})
+  set(OpenCV_INCLUDE_DIRS_CONFIGCMAKE "\"${OPENCV_INCLUDE_INSTALL_PATH}\" \"${OPENCV_INCLUDE_INSTALL_PATH}/opencv\"")
+else()
+  set(OpenCV_INCLUDE_DIRS_CONFIGCMAKE "\"\${OpenCV_INSTALL_PATH}/${OPENCV_INCLUDE_INSTALL_PATH}\" \"\${OpenCV_INSTALL_PATH}/${OPENCV_INCLUDE_INSTALL_PATH}/opencv\"")
+endif()
 
 if(USE_IPPICV)
   file(RELATIVE_PATH IPPICV_INSTALL_PATH_RELATIVE_CONFIGCMAKE "${CMAKE_INSTALL_PREFIX}" "${IPPICV_INSTALL_PATH}")