Merge pull request #13189 from alalek:cmake_options_warnings_update

cmake/OpenCVUtils.cmake

@@ -605,7 +605,9 @@ macro(OCV_OPTION variable description value)
       option(${variable} "${description}" ${__value})
     endif()
   else()
-    if(DEFINED ${variable} AND NOT OPENCV_HIDE_WARNING_UNSUPPORTED_OPTION)
+    if(DEFINED ${variable} AND "${${variable}}"  # emit warnings about turned ON options only.
+        AND NOT (OPENCV_HIDE_WARNING_UNSUPPORTED_OPTION OR "$ENV{OPENCV_HIDE_WARNING_UNSUPPORTED_OPTION}")
+    )
       message(WARNING "Unexpected option: ${variable} (=${${variable}})\nCondition: IF (${__condition})")
     endif()
     if(OPENCV_UNSET_UNSUPPORTED_OPTION)