Fix build with clang 3.8.0.

During a recent exp-run for bug 206074, it was found that x11-servers/x11rdp
gives errors with clang 3.8.0:

/wrkdirs/usr/ports/x11-servers/x11rdp/work/x11rdp_xorg71/build_dir/include/X11/Xtrans/Xtranssock.c:1027:50: error: reference to 'in6addr_any' is ambiguous

((struct sockaddr_in6 *)&sockname)->sin6_addr = in6addr_any;
                                                        ^

This is because Xtranssock.c attempts to redefine in6addr_any as a weak symbol,
which does not work with clang 3.8.0. As a fix, place the customized
in6addr_any definition between #ifdef FreeBSD guards.

PR: 207192
Submitted by: dim
Reviewed by: Koichiro IWAO <meta+ports@vmeta.jp> (maintainer)