* common.mk (ext/ripper/ripper.c): revert about srcdir and top_srcdir.

* common.mk (ext/ripper/ripper.c): use $(PWD) for Unix,
  $(MAKEDIR) for Windows.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48530 b2dd03c8-39d4-4d8f-98ff-823fe69b080e