Add a setlocal command to the beginning of build.bat. This is required

to deal with buildenv.bat properly, so that PATH (for example) doesn't
expand infintly. Per report from Joachim Wieland, patch from Magnus.

src/tools/msvc/build.bat

 @echo off
+SETLOCAL
 SET STARTDIR=%CD%
 if exist src\tools\msvc\buildenv.bat call src\tools\msvc\buildenv.bat
 if exist buildenv.bat call buildenv.bat