Use shared utf8proc library when USE_SYSTEM_UTF8PROC=1

Distribution policies strongly discourage statically linking to libraries in packages [1]  since security fixes and other bug fixes are not likely to be applied on time (and they would require a rebuild of the package anyway). People passing `USE_SYSTEM_UTF8PROC=1` are likely to prefer dynamic linking too, since I don't think many distributions provide static versions of utf8proc.

This patch seems to work but honestly it's worth checking a little more. :-)

1: https://fedoraproject.org/wiki/Packaging:Guidelines#Packaging_Static_Libraries_2