Fixing false alarm errors on windows build from a clean clone.

win/build-vs71.bat:
  Don't trigger 'could not find the file specified' errors when 
  running from a clean clone.
win/build-vs8.bat:
  Don't trigger 'could not find the file specified' errors when 
  running from a clean clone.

win/build-vs71.bat

 @echo off
 
-del cmakecache.txt
+if exist cmakecache.txt del cmakecache.txt
 copy win\vs71cache.txt cmakecache.txt
 cmake -G "Visual Studio 7 .NET 2003"
 copy cmakecache.txt win\vs71cache.txt

win/build-vs8.bat

 @echo off
 
-del cmakecache.txt
+if exist cmakecache.txt del cmakecache.txt
 copy win\vs8cache.txt cmakecache.txt
 cmake -G "Visual Studio 8 2005"
 copy cmakecache.txt win\vs8cache.txt