mruby/mruby — pull request #2553 by bggd (2014-08-22 07:59:52+00:00)
Title: Fix build failed on VS2012

use ```_copysign``` instead of ```copysign```.
VS2012 or older is only have ```_copysign``` and ```_copysignl```. (http://msdn.microsoft.com/en-us/library/0yafk1hc%28v=vs.110%29.aspx)