Makefile.sub: C99 mathematics functions

* win32/Makefile.sub (config.h): VC 2013 supports C99 mathematics
  functions.  [ruby-core:57981] [Bug #9044]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43398 b2dd03c8-39d4-4d8f-98ff-823fe69b080e