Don't use -Wno-uninitialized in math/

[Joseph Myers]

On Wed, 26 Aug 2015, Steve Ellcey wrote:

>       compiler does not see that it is set in all cases where it is
>       used, resulting in warnings that it may be used
>       uninitialized.  */

You need to move this comment up as well so it stays with the macro calls 
- and expand it to explain why it's at the top of the file.

-- 
Joseph S. Myers
joseph@codesourcery.com