ruby.h: HAVE_RB_SCAN_ARGS_OPTIONAL_HASH

* include/ruby/ruby.h (HAVE_RB_SCAN_ARGS_OPTIONAL_HASH): for
  rb_scan_args() optional hash feature.  [Bug #7861]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39335 b2dd03c8-39d4-4d8f-98ff-823fe69b080e