Add hex float support.

Add hex float support.

* modules/strtod (Depends-on): Add c-ctype.
(Link): Mention POW_LIB.
* lib/strtod.c (strtod): Recognize hex floats.  Don't allow
whitespace between 'e' and exponent.
* tests/test-strtod.c (main): Enable hex float tests.
* doc/posix-functions/strtod.texi (strtod): Document what gnulib
now provides.

Signed-off-by: Eric Blake <ebb9@byu.net>