Support __STDC_WANT_LIB_EXT2__ feature test macro

[Joseph Myers]

On Fri, 10 Jun 2016, Carlos O'Donell wrote:

> This looks good to me. I like the direction these changes are going in.

As there have been no further comments, I'll commit this patch after 
retesting (with the NEWS entry moved to the 2.25 section, of course).  
I'll then prepare (and commit as libm maintainer) patches for the 
__STDC_WANT_IEC_60559_BFP_EXT__ and __STDC_WANT_IEC_60559_FUNCS_EXT__ 
macros.

-- 
Joseph S. Myers
joseph@codesourcery.com