vpath.rb: hack for msys make

* tool/vpath.rb (VPath#def_options): hack for msys make, which
  converts a command line argument to non-msys command seems like a
  path list automagically.  [Bug #7710] [ruby-core:51489]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38886 b2dd03c8-39d4-4d8f-98ff-823fe69b080e