configure.in: fix for old shells

* configure.in (rm, shvar_to_cpp, unexpand_shvar): local is not
  available on old shells.
* configure.in (shvar_to_cpp): escape quotes for old shells.
  [Bug #7959] [Bug #8071]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39741 b2dd03c8-39d4-4d8f-98ff-823fe69b080e