Fix compilation on MacOS.
Fix compilation on MacOS.

MacOS is missing the char16_t type because the standard header uchar.h
is missing. This is documented here:

http://icu-project.org/apiref/icu4c/umachine_8h.html#a6bb9fad572d65b305324ef288165e2ac

This should be compatible on Linux as well.

seomoz/uri_parser,seomoz/uri_parser,seomoz/uri_parser