generic fenv wrapup

[Joel Sherrill]

Hi

We discussed a lot for this patch. I see FE_NOMASK_ENV isn't included.
When looking at that, the comment in /usr/include/bits/fenv.h on CentOS 7
has a subject verb agreement issue that leads me to question its intent.
 I would like someone to confirm that the glibc manual and this comment
mean the same thing:

ifd#f __USE_GNU
/* Floating-point environment where none of the exception is masked.  */
# define FE_NOMASK_ENV  ((const fenv_t *) -2)
#endif

And from the manual:

    If possible, the GNU C Library defines a macro FE_NOMASK_ENV which rep‐
       resents  an  environment  where every exception raised causes a trap
to
       occur.  You can test for this macro using #ifdef.  It is  defined
 only
       if  _GNU_SOURCE  is defined.

I think the manual version is clearer on intent.

Also.. there are three other GNU specific methods:

      #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <fenv.h>

       int feenableexcept(int excepts);
       int fedisableexcept(int excepts);
       int fegetexcept(void);

Should all those be added conditionally?

What else did we think needed to be done with the fenv generic code before
moving on to focusing on implementations?

--joel