gnulib: update to use new strtold module
* gnulib: Update to make the new strtold module available.
* bootstrap.conf: strtod is now a dependency of c-strtod,
which in turn is a dependency of cl-strtod. This treats
strtold and strtod similarly.
* gl/lib/cl-strtod.c: Adjust to assume strtold is available.
* tests/misc/sort-float.sh: Likewise.
* src/sort.c: Likewise.
(nan_compare): Adjust comment to indicate
we still have to init padding bits as per
https://sourceware.org/bugzilla/show_bug.cgi?id=13246