Here's a patch.

1. Not a huge fan of the too-similar underscore prefixed name. Choose a different name that is indicative of what this function does, rather than trying not to clash with the fnmatch() function name.
2. The casts are ugly- if you have to write a function anyway, make its declaration match the alpm_list_fn_cmp spec and drop the cast, please, doing them inside the function instead (something like _alpm_pkg_cmp).
3. Bonus- perhaps we should invert the groups loop so we only need one fnmatch comparator. So instead:
for(groups = alpm_pkg_get_groups(pkg); groups; groups = alpm_list_next(groups)) {
const char *group = alpm_list_getdata(groups);
if(alpm_list_find(handle->ignoregrp, group, fnmatch_cmp)) {
return(1);
}
}

Thank you for the review. Here's a second try.

Looks good now- just one minor tweak, you shouldn't cast away the const; pat and str are most definitely const. C also doesn't require he right hand side casts AFAIK; so you can probably do without those- the assignment enough is an explicit cast. Documentation updates would be great too.

const char *pat = pattern;
const char *str = string;

Documentation added and casting fixed.
The second patch is the same as the other one + the same idea applied to HoldPkg. I don't know, whether there's any need for wildcards in HoldPkg or not, but it wouldn't hurt either, i guess.

Implemented:

http://projects.archlinux.org/pacman.git/commit/?id=902305f1633298ff0ef7cd2bfbed0e91aae91646
http://projects.archlinux.org/pacman.git/commit/?id=10241a6d766fcebb3dbc930facb3c1b2c6ab4992