Fix build on OpenBSD's mips64 platforms.

OpenBSD's mips64 platforms (such as OpenBSD/sgi) do provide an get_fpc_csr(),
but lack the union fpc_csr. The configure script checks for get_fpc_csr() and
assumes union fpc_csr is also present (this is true for Irix, but not OpenBSD).
Merge pull request #29 from jasperla/master

Fix build on OpenBSD's mips64 platforms