DragonFlyBSD

The stack size of the main thread when pthread is used is limited to 4 MB (hard coded value).
This breaks, for example, the crystal compiler (www.crystal-lang.org).

The stack size of a process not using pthread is limited by RLIMIT_STACK. As soon as pthread is linked in,
the main thread's stack size is limited to 4 MB.

FreeBSD uses two environment variables LIBPTHREAD_BIGSTACK_MAIN and
LIBPTHREAD_SPLITSTACK_MAIN to control use of RLIMIT_STACK vs. the hard-coded
4MB and defaults to respect RLIMIT_STACK when none is specified.

This patch

https://leaf.dragonflybsd.org/~mneumann/patch-libthread_xu.diff

fixes the issue. Crystal compiles.