[[ Bug 13827 ]] Use UTF-8 strings when building iOS externals

Checking externalv1.cpp - it specifies kMCOptionAsString as returning a counted native string (ptr, length) pair. I think here you do want to do kMCOptionAsUTF8CString here - then use stringWithCString: encoding NSUTF8StringEncoding later on.

(Of course, if kMCOptionAsString is producing UTF8 - something is wrong there ;))