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.
distance: enforce english locale for numbers given in HWLOC_GROUPING_ACCURACY
tests/xml: Fix XML distance-grouping with icc by relaxing the accuracy

icc aggregates distance matrices into slightly different floats,
so enforce 0.01 accuracy to make things work.
Add may_alias attribute support and silence some safe aliasing in the XML backends