Since commit 70aab459, the msvc and MinGW builds have relied on
the built-in implementation of ntohl() and htonl(), rather than
linking the wsock32 library. The new index manipulation code now
calls ntohs()/htons() in addition to ntohl()/htonl(), so we need
to provide a built-in implementation of the 16-bit functions.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>