configure.ac:
Set flags to build Boehm GC with threads support and parallel
marking.
Prevent use of "__thread" storage class specifier on OpenBSD;
it does not work.
Enable shared libraries on OpenBSD.
README.OpenBSD:
New file.
.README.in:
README.md:
Mention README.OpenBSD.
.nocopyright:
Add README.OpenBSD.
