net-p2p/amule: Disable C++17 error about usage of the register keyword

Instead of forcing the port to build with -std=c++11 in all FreeBSD
versions, just pass -Wno-register when the port is being built with a
Clang release that defaults to C++17 mode (which does not have the
register keyword by default).

Approved by: eduardo (maintainer)
Differential Revision: https://reviews.freebsd.org/D40939