configure.in: don't substitute exec_prefix itself

* configure.in (shvar_to_cpp): do not substitute exec_prefix itself
  with RUBY_EXEC_PREFIX, which cause recursive definition.
  [ruby-core:52296] [Bug #7860]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39273 b2dd03c8-39d4-4d8f-98ff-823fe69b080e