* ext/digest/*/*.[ch]: include ruby.h before digest.h to avoid
includeing ext/digest/extconf.h. [Bug #3231]
https://msdn.microsoft.com/library/36k2cdd4.aspx
* ext/digest/*/extconf.rb: remove ext/digest from include search path
to avoid confusion of cl.exe.
* ext/digest/*/*.[ch]: explicitly specify def.h's path.
merge revision(s) 52694: [Backport #3231]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@52797 b2dd03c8-39d4-4d8f-98ff-823fe69b080e