'win32/*.bat' do not handle SRCROOT long path names with spaces correctly

as an example, from a DOS box, type:
  set SRCROOT=c:\Xiph CVS\
  call build_vorbis_static.bat
produces the error:
  cvs==. was unexpected at this time.
and does *not* set %SRCROOT% to any value
resolution:
  replace the line:
    if .%SRCROOT%==. set SRCROOT=c:\src
  with the line:
    if ".%SRCROOT%"=="." set SRCROOT=c:\src