build: fix ceph_statx check when configured with libcephfs_dir

build: fix ceph_statx check when configured with libcephfs_dir

When configured with a custom libcephfs_dir, the ceph_statx check fails
to link. This is due to the location of the ceph-common dependency,
which is installed under a ceph subdirectory.

ceph/build > make DESTDIR=./inst install
...
ceph/build > find inst/|grep -e /libcephfs -e /libceph-common
inst/usr/local/lib64/ceph/libceph-common.so.0
inst/usr/local/lib64/ceph/libceph-common.so
inst/usr/local/lib64/libcephfs.so.2.0.0
inst/usr/local/lib64/libcephfs.so.2
inst/usr/local/lib64/libcephfs.so
inst/usr/local/include/cephfs/libcephfs.h

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Jeff Layton <jlayton@samba.org>
Autobuild-User(master): David Disseldorp <ddiss@samba.org>
Autobuild-Date(master): Thu Jul 20 23:02:27 CEST 2017 on sn-devel-144