BuildInstaller.bat: download WiX 3.14 if it is not present

Src/Setup/BuildInstaller.bat

@@ -36,6 +36,16 @@ md Temp
 	@set /a "CS_VERSION_NUM=%%A<<24|%%B<<16|%%C"
 )
 
+@REM Check if WiX is available in PATH
+@candle --help 1> nul 2>&1
+if ERRORLEVEL 1 (
+	@echo --- Downloading WiX...
+	@curl -s -L https://wixtoolset.org/downloads/v3.14.0.4118/wix314-binaries.zip -o Temp\wix314-binaries.zip > nul
+	@7z x Temp\wix314-binaries.zip -oTemp\WiX -r > nul
+	@if ERRORLEVEL 1 exit /b 1
+)
+@if exist "Temp\WiX" set PATH=%PATH%;%CD%\Temp\WiX
+
 REM ********* Build x86 MSI
 echo --- x86 MSI
 candle Setup.wxs -nologo -out Temp\Setup32.wixobj -ext WixUIExtension -ext WixUtilExtension -dx64=0 -dARM64=0 -dCS_LANG_FOLDER=%CS_LANG_FOLDER% -dCS_LANG_NAME=%CS_LANG_NAME%