ceph: Remove unavailable option with clang

Some options are unavailable in clang. (-Wstrict-null-sentinel,
-rdynamic)

They were reintroduced by this commit
https://github.com/ceph/ceph/commit/6949d221ada12e6be4b5ebf94b58edd17288fb16

This patch fixes it.

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
Merge pull request #734 from kri5/master

ceph: Remove unavailable option with clang

Reviewed-by: Sage Weil <sage@inktank.com>