Don't ever define __USE_GNU directly

[Jakub Jelinek]

Hi!

fpu-glibc.h has:

#define __USE_GNU
#ifdef HAVE_FENV_H
#include <fenv.h>
#endif

This is wrong, __USE_GNU is glibc internal macro that shouldn't be ever
defined by apps.
The way to select GNU feature set in glibc headers is to define _GNU_SOURCE,
either before including first include header in the source .c/.C file,
or by defining it on the command line (-D_GNU_SOURCE).

	Jakub