* file.c (stat_birthtime): add birthtime support [Feature #9647]

* file.c (rb_stat_birthtime): add File::Stat.birthtime

* file.c (rb_file_s_birthtime): add File.birthtime

* file.c (rb_file_birthtime): add File#birthtime

* configure.in: check struct stat.st_birthtimespec.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46047 b2dd03c8-39d4-4d8f-98ff-823fe69b080e