propername-lite: lighten it up some more

propername-lite: lighten it up some more

This should help GNU diffutils avoid some Gnulib modules.
* lib/propername-lite.c: Do not include c-strcase.h, localcharset.h.
Include <uchar.h> and mbrtowc32 if available.
Use __has_include to detect this; that should be good enough nowadays.
Use native mbrtoc32 instead of Gnulib replacement; that should
be good enough.  2nd arg is now possibly unused.
Use mbrtoc32, if available, to determine whether UTF-8 is being used,
to avoid dependencies.
* modules/propername-lite (Depends-on):
Remove localcharset, c-strcasecmp.