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

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13250

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Jeff Layton <jlayton@samba.org>
(cherry picked from commit ff7df3d3f5259362a6bb6780d6b532e57e89681d)

Autobuild-User(v4-6-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-6-test): Tue Feb 13 13:36:13 CET 2018 on sn-devel-144