Build system: Add LT_LDRULE_shlib_missing

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>