* lib/exclude.c (FNM_EXTMATCH): Define if system does not.

* lib/exclude.c (FNM_EXTMATCH): Define if system does not.
Verify that it doesn't overlap with our flags.
(fnmatch_no_wildcards): Don't use strcasecmp or strncasecmp, which
do not have the desired effect in multibyte locales; instead, use
mbscasecmp.
* modules/exclude (Depends-on): Depend on mbscasecmp, not strcase.
Add dependency on xalloc.  Depend on fnmatch, not fnmatch-gnu, since
we don't require GNU fnmatch ourselves (if our users require it, they
should do so explicitly).