strtod: make it more-accurate typically, and don't require libm

strtod: make it more-accurate typically, and don't require libm

* lib/strtod.c (_GL_ARG_NONNULL): Remove; no longer needed.
Include limits.h.  Don't include string.h.
(HAVE_LDEXP_IN_LIBC, HAVE_RAW_DECL_STRTOD): Define to 0 if not defined.
(locale_isspace): New function, so that no casts are needed to
check whether *s is a space.
(ldexp): Provide an unused dummy if not available.
(scale_radix_exp, parse_number, underlying_strtod): New functions.
(strtod): Use them.  This implementation prefers to use the
underlying strtod if available, falling back on our own code
only to fix known bugs.  This is more likely to produce an
accurate result.  Also, it avoids the use of libm functions.
* m4/strtod.m4 (gl_FUNC_STRTOD): Don't invoke _AC_LIBOBJ_STRTOD;
no longer needed.  Invoke AC_LIBOBJ([strtod]); don't know why this
was absent, but it caused a test failure with coreutils.
(gl_PREREQ_STRTOD): Check wither ldexp can be used without linking
with libm.
* modules/strtod (Makefile.am, Link): libm is no longer needed.
* modules/strtod-tests (Makefile.am): Likewise.