maint: sort THANKS using our own gear

maint: sort THANKS using our own gear

At least 'sort' on openSUSE/Fedora have a bug in the case-folding code
of their I18N downstream patch which leads to wrong sort results,
e.g. "Dániel" coming after "Dylan".

* Makefile.am (THANKS): Sort the final contributor list using our
own sort implementation - as others may result in a different order;
add a FIXME comment to remove this again once common platforms have
a functional 'sort -f'.  Add '-k1,1' for a better sort order.
While at it, save a grep and perl call to prepare the list from
'THANKS.in' by doing all in the first perl call.