Makefile.sub: fix nextafter

* win32/Makefile.sub (MISSING, CONFIG_H): msvcr120.dll provides
  nextafter() as well as other mathematic functions.  finite() and
  isnan() used in missing/nextafter.c are not by older runtimes.
  [Feature #9834]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45992 b2dd03c8-39d4-4d8f-98ff-823fe69b080e