imgcodecs: disable OpenEXR in runtime for 3rdparty source code

- builtin OpenEXR source code is outdated
- external OpenEXR distributions are allowed

cmake/OpenCVFindLibsGrfmt.cmake

@@ -240,6 +240,7 @@ if(WITH_OPENEXR)
     set(OPENEXR_LIBRARIES IlmImf)
     add_subdirectory("${OpenCV_SOURCE_DIR}/3rdparty/openexr")
     if(OPENEXR_VERSION)  # check via TARGET doesn't work
+      set(BUILD_OPENEXR ON)
       set(HAVE_OPENEXR YES)
     endif()
   endif()