Use double backslashes for the path separators to workaround Microsoft

Use double backslashes for the path separators to workaround Microsoft
NMAKE brain damaging.  Sometimes backslashes are treated as a quoting
characters, and sometimes not, with no rhyme or reason that I can
determine.  Fortunatelly double backslashes in pathanmes don't seem to
hurt (for the times when the backslash isn't treated as a quoating
character).  Whoever decided Windows should use backslash as a path
separator should be shot.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@10575 dc483132-0cff-0310-8789-dd5450dbe970