hwloc_strncasecmp: don't fallback to strncmp when strncasecmp is missing

This fallback breaks some internals where we assume that hwloc_strncasecmp
is really case-insensitive. Reimplement a basic strncasecmp instead.

This is used on HP-UX, and with icc when it fails to properly check whether
strcasecmp is available.

Thanks to Nick Papior Andersen for reporting the problem.

(cherry picked from commit c9dcf65977cb170b802199403e8068b8122ab151)