dir.c: define O_CLOEXEC for older systems

	SuSE 10 has openat(), but not O_CLOEXEC

	Reported-by: wangpeiwen

	[ruby-core:87591] [Bug #14864]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@66878 b2dd03c8-39d4-4d8f-98ff-823fe69b080e