getdate.y: avoid locale-dependent date parsing failure

getdate.y: avoid locale-dependent date parsing failure

In Turkish locales, getdate would fail to recognize keywords
containing a lowercase "i".  The solution is not to rely on
locale-sensitive case-conversion.
* lib/getdate.y: Include <c-ctype.h> rather than <ctype.h>.
(lookup_word): Use c_toupper in place of toupper.
(yylex, get_date): Use c_ prefixed variants of isspace and isalpha, too.
Reported by Vefa Bicakci <bicave@superonline.com> in
<http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/14184>.
* modules/getdate (Depends-on): Add c-ctype.