build: minor changes for cmake 3.30 and some cleanup

cmake/OpenCVDetectCXXCompiler.cmake

@@ -83,6 +83,10 @@ if(NOT DEFINED CMAKE_SIZEOF_VOID_P
     AND NOT OPENCV_SUPPRESS_MESSAGE_MISSING_CMAKE_SIZEOF_VOID_P)
   message(WARNING "OpenCV: CMAKE_SIZEOF_VOID_P is not defined. Perhaps CMake toolchain is broken")
 endif()
+if(NOT CMAKE_SIZEOF_VOID_P GREATER 0)
+  message(FATAL_ERROR "CMake fails to determine the bitness of the target platform.
+  Please check your CMake and compiler installation. If you are cross-compiling then ensure that your CMake toolchain file correctly sets the compiler details.")
+endif()
 
 message(STATUS "Detected processor: ${CMAKE_SYSTEM_PROCESSOR}")
 if(OPENCV_SKIP_SYSTEM_PROCESSOR_DETECTION)
@@ -156,8 +160,10 @@ elseif(MSVC)
     set(OpenCV_ARCH "ARM")
   elseif("${CMAKE_SIZEOF_VOID_P}" STREQUAL "8")
     set(OpenCV_ARCH "x64")
+  elseif("${CMAKE_SIZEOF_VOID_P}" STREQUAL "4")
+    set(OpenCV_ARCH "x86")
   else()
-    set(OpenCV_ARCH x86)
+    message(FATAL_ERROR "Failed to determine system architecture")
   endif()
 
   if(MSVC_VERSION EQUAL 1400)

cmake/OpenCVDownload.cmake

@@ -40,11 +40,14 @@ file(REMOVE "${OPENCV_DOWNLOAD_WITH_WGET}")
 ocv_check_environment_variables(OPENCV_DOWNLOAD_MIRROR_ID)
 
 function(ocv_init_download_mirror)
+  if(NOT GIT_FOUND)
+    return()
+  endif()
   if(NOT DEFINED OPENCV_DOWNLOAD_MIRROR_ID)
     # Run `git remote get-url origin` to get remote source
     execute_process(
       COMMAND
-        git remote get-url origin
+        ${GIT_EXECUTABLE} remote get-url origin
       WORKING_DIRECTORY
         ${CMAKE_SOURCE_DIR}
       RESULT_VARIABLE