So, someone would need to do one of these:
- Port libmicrohttpd to Windows proper.
- Fix PlibC to work on not only just 32-bit Windows.
- Port BFGMiner to support another HTTP server library (I'm not aware of any decent alternatives, unfortunately).
Have a looksie at http://lists.gnu.org/archive/html/libmicrohttpd/2013-03/msg00007.html
Sounds like someone already patched plibc to work on x64 and add utf-8 support but his patch was ignored/forgotten. At least he posted a git where it can be had.