[build] Fix building on OpenBSD 5.4

OpenBSD 5.4 seems to generate dynamically linked binaries by default,
which breaks our build process.  Fix by forcing the linker to always
create static binaries.

Reported-by: Jiri B <jirib@devio.us>
Tested-by: Jiri B <jirib@devio.us>
Signed-off-by: Michael Brown <mcb30@ipxe.org>