Fix VS 2012 build

VS2012 misses several mathf functions that are supported starting with vs2013 (http://blogs.msdn.com/b/vcblog/archive/2013/07/19/c99-library-support-in-visual-studio-2013.aspx).

Philippe
It's probably preferable to code this as

   #if !defined(_MSC_VER) || _MSC_VER >= 1800