Fix build on openbsd 7.0 (#929)

Fix build on openbsd 7.0 (#929)

OpenBSD doesn't offer CPU_SET and CPU_ISSET. Implement their stubs as
inline functions to give the compiler proper hints about arguments (non)
use.

We have a const-correctness bug in std::unordered_map when supplying an
allocator that OpenBSD is strict about. Fix it.

Update buildtest.sh to try and use relative paths first. This prevents
autoconf complaining and failing if the directory path includes
characters from an unsafe set.