* configure.in: Check nextafter() availability.

* include/ruby/missing.h (nextafter): New optional declaration.

* missing/nextafter.c: New file.

* numeric.c: Float#next_float and Float#prev_float implemented.

   [ruby-core:62562] [Feature #9834]



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45982 b2dd03c8-39d4-4d8f-98ff-823fe69b080e