games/dangerdeep: Unbreak build on FreeBSD >= 12.1

src/subsim.cpp:1739:2: error: reference to 'thread' is ambiguous

thread::auto_ptr<music> mmusic(new music(use_sound));
^

src/thread.h:37:7: note: candidate found by name lookup is 'thread'
class thread

^

/usr/include/c++/v1/__threading_support:397:24: note: candidate found by name lookup is 'std::__1::thread'
class _LIBCPP_TYPE_VIS thread;

^

2 warnings and 1 error generated.

http://beefy6.nyi.freebsd.org/data/121amd64-default/529299/logs/errors/dangerdeep-0.3.0_14.log