c-strtod, c-strtold: Use the bug fixes for strtod, strtold.

c-strtod, c-strtold: Use the bug fixes for strtod, strtold.

* lib/stdlib.in.h (GNULIB_defined_strtod_function,
GNULIB_defined_strtold_function): New macros.
* lib/c-strtod.c (HAVE_GOOD_STRTOD_L): New macro.
(STRTOD): Ignore HAVE_C99_STRTOLD.
(c_locale): Don't define it on platforms where strtod_l/strtold_l is
deemed buggy. But do use it on platforms where uselocale exists and is
usable.
(C_STRTOD): Don't use STRTOD_L on platforms where strtod_l/strtold_l is
deemed buggy. On platforms where uselocale exists and is usable, use
uselocale and strtod/strtold.
* m4/c-strtod.m4 (gl_C99_STRTOLD): Remove macro.
(gl_C_STRTOD): Require gt_FUNC_USELOCALE.
(gl_C_STRTOLD): Likewise. Define HAVE_C99_STRTOLD unconditionally.
* modules/c-strtod (Files): Add m4/intl-thread-locale.m4.
(Depends-on): Add strtod.
* modules/c-strtold (Files): Add m4/intl-thread-locale.m4.
(Depends-on): Add strtold.