Defect in XBD fenv.h

[Geoff Clare <yyy@xxxxxxxxxxxxx>]

@ page 226 line 7956-7963 section fenv.h objection [gwc fenv rounding]

Problem:

Defect code :  1. Error

XBD ERN 48 corrects a mismatch between POSIX and the C Standard over
the requirements for floating-point exception macros in <fenv.h>.
A similar problem exists for the rounding macros.

Action:

On line 7956 change:

    "shall define the following"

to:

    "shall define each of the following" 

After line 7963 add:

    [MX]If the implementation supports the IEC 60559 Floating-Point
    option, all four constants shall be defined. [/MX]
    Additional implementation-defined rounding directions with
    constants beginning with FE_ and an uppercase letter may also
    specified by the implementation.

Note to the editor: if the use of the word "constants" on the
<fenv.h> page changes as a result of the macro/constant issue
raised on the reflector, then the use of "constants" in the
above new text, and in the new text added by XBD ERN 48, should
change to match.