Use Gnulib's `isnan' and `isinf' modules.

Use Gnulib's `isnan' and `isinf' modules.

This updates Gnulib to v0.0-4219-g84cdd8b.

* m4/gnulib-cache.m4: Add `isinf' and `isnan'.

* configure.ac: Remove checks for `floatingpoint.h', `ieeefp.h', and
  `nan.h'.

* libguile/gen-scmconfig.c (main): Remove definitions of
  `SCM_HAVE_FLOATINGPOINT_H', `SCM_HAVE_IEEEFP_H', and
  `SCM_HAVE_NAN_H'.

* libguile/numbers.c (isnan)[SCO && !HAVE_ISNAN]: Remove.
  (isinf)[SCO && !HAVE_ISINF]: Remove.
  (xisinf, xisnan): Remove.  Change callers to use `isinf' and `isnan'.
  (guile_ieee_init): Remove the `defined HAVE_ISINF' and `define
  HAVE_ISNAN' conditions.

* libguile/numbers.h: Remove code conditional on
  `SCM_HAVE_FLOATINGPOINT_H', `SCM_HAVE_IEEEFP_H', or `SCM_HAVE_NAN_H'.