can be built for one place and installed in another

these syscalls will be available in osx 10.10

syscalls added (openat, access, accessat, faccessat, etc.)

Use __thread for compatibility with gcc <= 4.7.

Use __thread in place of thread_local.  This is necessary to compile using gcc
version 4.7.2, the version of gcc present in Debian stable (wheezy).

It's way faster than pthreads, but it's deprecated and not always available.

Use Linux convention that reserves top 4K of return values for errors;
negate errno to produce value in that range.

Since apparently ucontext.h is deprecated on Mac OS.

Why must they think different?

Instead, search {install prefix}/riscv-elf/bin/.
Thank you, Palmer.

Current implementations automatically zero the tohost register when the host
reads it.  This is overkill, since we provide an atomic swap operation, which
can accomplish the same thing without an additional special case.  Future
implementations will not support the auto-reset behavior.