BUG: #1707: fix OpenCV compilation with GCC 7

SuperBuild/patches/OPENCV/opencv-2-compiler-check-all.diff

+diff -burN OPENCV-orig/cmake/OpenCVDetectCXXCompiler.cmake OPENCV/cmake/OpenCVDetectCXXCompiler.cmake
+--- OPENCV-orig/cmake/OpenCVDetectCXXCompiler.cmake	2018-09-05 09:39:36.254808812 +0200
++++ OPENCV/cmake/OpenCVDetectCXXCompiler.cmake	2018-09-05 09:44:19.273777344 +0200
+@@ -77,6 +77,10 @@
+ 
+   # Split the three parts:
+   string(REGEX MATCHALL "[0-9]+" CMAKE_OPENCV_GCC_VERSIONS "${CMAKE_GCC_REGEX_VERSION}")
++  list(LENGTH CMAKE_OPENCV_GCC_VERSIONS CMAKE_OPENCV_GCC_VERSIONS_LENGTH)
++  if(${CMAKE_OPENCV_GCC_VERSIONS_LENGTH} LESS 2)
++    string(REGEX MATCHALL "[0-9]+" CMAKE_OPENCV_GCC_VERSIONS "${CMAKE_CXX_COMPILER_VERSION}")
++  endif()
+ 
+   list(GET CMAKE_OPENCV_GCC_VERSIONS 0 CMAKE_OPENCV_GCC_VERSION_MAJOR)
+   list(GET CMAKE_OPENCV_GCC_VERSIONS 1 CMAKE_OPENCV_GCC_VERSION_MINOR)