Fix MinGW architecture detection

Fix typo that would always lead to detection of x86 for MinGW
builds in the OpenCVConfig.cmake file.

cmake/OpenCVConfig.cmake

@@ -87,7 +87,7 @@ elseif(MINGW)
   set(OpenCV_RUNTIME mingw)
 
   execute_process(COMMAND ${CMAKE_CXX_COMPILER} -dumpmachine
-                  OUTPUT_VARIABLE OPENCV_GCC_TARGET_MACHINE
+                  OUTPUT_VARIABLE CMAKE_OPENCV_GCC_TARGET_MACHINE
                   OUTPUT_STRIP_TRAILING_WHITESPACE)
   if(CMAKE_OPENCV_GCC_TARGET_MACHINE MATCHES "64")
     set(MINGW64 1)