Add build.bat for Windows MSYS2 build

build.bat

@@ -0,0 +1,23 @@
+@echo off
+echo uTun Build Script for MSYS2 UCRT64
+echo.
+
+REM Default MSYS2 path
+set MSYS2_PATH=C:\msys64
+
+REM Use environment variable if set
+if not "%MSYS2_ROOT%"=="" set MSYS2_PATH=%MSYS2_ROOT%
+if not "%MSYS2%"=="" set MSYS2_PATH=%MSYS2%
+
+REM Check if msys2_shell.cmd exists
+if not exist "%MSYS2_PATH%\msys2_shell.cmd" (
+    echo MSYS2 not found at %MSYS2_PATH%
+    echo Please install MSYS2 from https://www.msys2.org/
+    echo or set MSYS2_ROOT environment variable.
+    pause
+    exit /b 1
+)
+
+REM Launch MSYS2 UCRT64 shell and run build script
+echo Starting MSYS2 UCRT64 environment...
+call "%MSYS2_PATH%\msys2_shell.cmd" -ucrt64 -here -c "bash build.sh > build_win.log 2>&1"