make use of __builtin_isnan

[Bruno Haible]

GCC 4.0 or newer also has a built-in for isnan(double). This patch makes use of
it.

2008-01-26  Bruno Haible  <address@hidden>

        * m4/isnand.m4 (gl_FUNC_ISNAND_NO_LIBM): Test the GCC >= 4.0 built-in.
        * lib/isnand.h (isnand): Use the GCC >= 4.0 built-in.

*** lib/isnand.h.orig   2008-01-26 15:34:00.000000000 +0100
--- lib/isnand.h        2008-01-26 15:33:42.000000000 +0100
***************
*** 17,23 ****
  #if HAVE_ISNAND_IN_LIBC
  /* Get declaration of isnan macro.  */
  # include <math.h>
! # ifdef isnan
  #  undef isnand
  #  define isnand(x) isnan ((double)(x))
  # endif
--- 17,27 ----
  #if HAVE_ISNAND_IN_LIBC
  /* Get declaration of isnan macro.  */
  # include <math.h>
! # if __GNUC__ >= 4
!    /* GCC 4.0 and newer provides three built-ins for isnan.  */
! #  undef isnand
! #  define isnand(x) __builtin_isnan ((double)(x))
! # else
  #  undef isnand
  #  define isnand(x) isnan ((double)(x))
  # endif
*** m4/isnand.m4.orig   2008-01-26 15:34:00.000000000 +0100
--- m4/isnand.m4        2008-01-26 15:32:13.000000000 +0100
***************
*** 1,4 ****
! # isnand.m4 serial 1
  dnl Copyright (C) 2007-2008 Free Software Foundation, Inc.
  dnl This file is free software; the Free Software Foundation
  dnl gives unlimited permission to copy and/or distribute it,
--- 1,4 ----
! # isnand.m4 serial 2
  dnl Copyright (C) 2007-2008 Free Software Foundation, Inc.
  dnl This file is free software; the Free Software Foundation
  dnl gives unlimited permission to copy and/or distribute it,
***************
*** 12,19 ****
      [gl_cv_func_isnand_no_libm],
      [
        AC_TRY_LINK([#include <math.h>
                     double x;],
!                   [return isnan (x);],
          [gl_cv_func_isnand_no_libm=yes],
          [gl_cv_func_isnand_no_libm=no])
      ])
--- 12,26 ----
      [gl_cv_func_isnand_no_libm],
      [
        AC_TRY_LINK([#include <math.h>
+                    #if __GNUC__ >= 4
+                    # undef isnand
+                    # define isnand(x) __builtin_isnan ((double)(x))
+                    #else
+                    # undef isnand
+                    # define isnand(x) isnan ((double)(x))
+                    #endif
                     double x;],
!                   [return isnand (x);],
          [gl_cv_func_isnand_no_libm=yes],
          [gl_cv_func_isnand_no_libm=no])
      ])