strtold: New module.

strtold: New module.

* lib/stdlib.in.h (strtold): New declaration.
* lib/strtold.c: New file.
* lib/strtod.c: Consider USE_LONG_DOUBLE.
(STRTOD, LDEXP, HAVE_UNDERLYING_STRTOD, DOUBLE, MIN, MAX, L_,
USE_LDEXP): New macros.
(LDEXP, scale_radix_exp, parse_number, STRTOD): Adapt for
USE_LONG_DOUBLE.
(underlying_strtod): Remove function. Replace with some macros.
Re-add the code for a missing underlying function that was removed on
2013-02-19.
* m4/strtold.m4: New file.
* m4/stdlib_h.m4 (gl_STDLIB_H): Test whether strtold is declared.
(gl_STDLIB_H_DEFAULTS): Initialize GNULIB_STRTOLD, HAVE_STRTOLD,
REPLACE_STRTOLD.
* modules/stdlib (Makefile.am): Substitute GNULIB_STRTOLD, HAVE_STRTOLD,
REPLACE_STRTOLD.
* modules/strtold: New file.
* doc/posix-functions/strtold.texi: Document the new module.