internal.h: use SIGNEDNESS_OF_TIME_T
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39956 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
internal.h: use SIGNEDNESS_OF_TIME_T
check_signedness of mkmf.rb.
SIGNEDNESS_OF_TIME_T.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39956 b2dd03c8-39d4-4d8f-98ff-823fe69b080e