Fix target platform detection for x64 MinGW

cmake/OpenCVDetectCXXCompiler.cmake

@@ -93,13 +93,13 @@ elseif(CMAKE_COMPILER_IS_GNUCXX)
         execute_process(COMMAND ${CMAKE_CXX_COMPILER} -dumpmachine
                   OUTPUT_VARIABLE CMAKE_OPENCV_GCC_TARGET_MACHINE
                   OUTPUT_STRIP_TRAILING_WHITESPACE)
-        if(CMAKE_OPENCV_GCC_TARGET_MACHINE MATCHES "64")
+        if(CMAKE_OPENCV_GCC_TARGET_MACHINE MATCHES "amd64|x86_64|AMD64")
             set(MINGW64 1)
         endif()
     endif()
 endif()
 
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "amd64.*|x86_64.*|AMD64.*" OR CMAKE_GENERATOR MATCHES "Visual Studio.*Win64")
+if(MINGW64 OR CMAKE_SYSTEM_PROCESSOR MATCHES "amd64.*|x86_64.*|AMD64.*" OR CMAKE_GENERATOR MATCHES "Visual Studio.*Win64")
     set(X86_64 1)
 elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "i686.*|i386.*|x86.*|amd64.*|AMD64.*")
     set(X86 1)