Explicitly build with -std=gnu++11.

This is a workaround for FreeBSD 10, whose libc++ does not have an overload for
C++14's `operator delete(void*, size_t)'.

Since GCC 6 uses -std=gnu++14 by default, linking fails when a dependency (such
as taglib) is built with clang and lacking the overload. FreeBSD >= 11 is fine.

PR: 219489
Approved by: portmgr (blanket approval)