26_numerics/c99_classification_macros_c++.cc fails on cygwin

[Gabriel Dos Reis]

"Billinghurst, David (CRTS)" <David.Billinghurst@riotinto.com> writes:

| I have made a little more progress on this.
|  - cygwin has full C99 support in <math.h>,
|  - but not in <stdlib.h>.
| so that _GLIBCPP_USE_C99 is not defined
| 
| This means that the libstdc++-v3/include/c_std/std_cmath.h
| doesn't undefine the C99 FP macros.
| 
| Details below.  Where to from here?

Thanks for the detective work.

In the past, we used to detect those features on case by case basis.
People complained about the slowness of the scheme, thus we switched
to all-or-nothing scheme.  

-- Gaby