generate_res.bat: automatic detection if windres.exe is available

build/VS2010/zstd/generate_res/generate_res.bat

@@ -1,3 +1,10 @@
+@echo off
 REM http://stackoverflow.com/questions/708238/how-do-i-add-an-icon-to-a-mingw-gcc-compiled-executable
-REM copy "c:\Program Files (x86)\Windows Kits\8.1\Include\um\verrsrc.h" .
+
-windres -I ..\..\..\..\lib -O coff -I . -i ..\zstd.rc -o zstd.res
+where /q windres.exe
+IF ERRORLEVEL 1 (
+    ECHO The windres.exe is missing. Ensure it is installed and placed in your PATH.
+    EXIT /B
+) ELSE (
+    windres.exe -I ..\..\..\..\lib -O coff -I . -i ..\zstd.rc -o zstd.res
+)