etc.c: try confstr on Mac OS X

* ext/etc/etc.c (etc_systmpdir): try user temporary directory by
  confstr() on Mac OS X.
  c.f. http://www.opensource.apple.com/source/ruby/ruby-104/patches/ext_etc_etc.c.diff

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47826 b2dd03c8-39d4-4d8f-98ff-823fe69b080e