Build system: Add LT_LDRULE_shlib_missing

Add a new LT_LDRULE which can be used to build shared libraries which
may not contain all of the symbols listed in their .sym files. This
allows us to build a library like roken, where different symbols
will be present on different operating systems.

Change-Id: I82e0df49131aa7184360130547c82052a6dd704b
Reviewed-on: http://gerrit.openafs.org/8901
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
OpenBSD: Add config headers for new(er) OS versions.

The OS-specific config header files have not kept up with the
newer releases of OpenBSD. This patch corrects that by adding
header files for OpenBSD versions from 4.9 to 5.3 (the newest
version currently under development).

Change-Id: I5e233cc7b96be2c1c52824653c42d3c6d77d6f50
Reviewed-on: http://gerrit.openafs.org/8927
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>