extconf.rb do not need to check if headers exist under separated
directory. [ruby-dev:35437]
* include/{ruby,rubyio,rubysig}.h, include/ruby/intern.h: use
"ruby/..." instead of <ruby/...>.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@18039 b2dd03c8-39d4-4d8f-98ff-823fe69b080e