Makefile.sub: SIGNEDNESS_OF_TIME_T

* win32/Makefile.sub (config.h): add SIGNEDNESS_OF_TIME_T for
  TIMET_MAX and TIMET_MIN.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39966 b2dd03c8-39d4-4d8f-98ff-823fe69b080e