Add --with-system-libunistring?

[Paolo Bonzini]

libunistring is starting to be included in many GNU/Linux distributions, 
but still most programs using gnulib's unicode modules have a nontrivial 
amount of code that could be eliminated if the system libunistring was 
used instead.

So maybe we could do the following:

1) change libunistring to define an automake conditional if libunistring 
is found;

2) change modules/uni* so that if the conditional is true, then the 
modules are left out.

This way, packages that wish to not add another dependency can still 
include a statically-linked copy via gnulib, but will rely on the system 
library when available.

Paolo