Update Gnulib; add new modules; remove `round' module.

This updates Gnulib to v0.0-4951-g6ff7b70.

* m4/gnulib-cache.m4: Add floor, ceil, frexp, and ldexp.  Add wchar as
  an explicit dependency; it had been present as an indirect dependency
  before, but no longer.  Remove round, which I had requested earlier,
  but turned out to be unnecessary.

lib/isnand-nolibm.h

@@ -0,0 +1,33 @@
+/* Test for NaN that does not need libm.
+   Copyright (C) 2007-2011 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#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
+#else
+/* Test whether X is a NaN.  */
+# undef isnand
+# define isnand rpl_isnand
+extern int isnand (double x);
+#endif