Missing HAVE_SIGNBIT, HAVE_SIGNBITF, and HAVE_SIGNBITL break build

[Aaron W. LaFramboise]

With this patch:
http://gcc.gnu.org/ml/libstdc++/2004-05/msg00062.html
libstdc++-v3 no longer builds for me on i686-pc-mingw32 as part of a GCC 
bootstrap.

autoheader: warning: missing template: HAVE_SIGNBIT
autoheader: Use AC_DEFINE([HAVE_SIGNBIT], [], [Description])
autoheader: warning: missing template: HAVE_SIGNBITF
autoheader: warning: missing template: HAVE_SIGNBITL

There does not seem to be any template for signbit, only for __signbit 
(HAVE___SIGNBIT).

Aaron W. LaFramboise