Since very few people run xfs with dmapi enabled it seem kind of silly
to force the install of the dmapi utils/libraries just for xfs dump/restore.

This patch stubs out the Hsm functions and adjusts the autoconf
to not fail in the case of missing dmapi headers/libs.

If somebody does trie to do a dump with the -a option a ERROR message
is issued, the dump does continue? this might not be the desired behavior.