buffer: make buffer::exception classes undefined in dynamic objects

buffer: make buffer::exception classes undefined in dynamic objects

On OSX, if the an exception class is declared and defined in header file,
but it ends up being compiled as private symbols in different binaries.
The exception handling code will take the two compiled exception classes
as different types! In our case, the one in libcls_xxx.so and the one is
ceph-osd are considered as different classes, thus the try-catch statement
fails to work.

The fix is force buffer::exception classes undefined in libcls_xxx.so. The
ibcls_xxx.so are compiled with '-undefined dynamic_lookup' option. when
it is loaded into ceph-osd, buffer::exception classes in ceph-osd will be
used.

Signed-off-by: Yan, Zheng <zyan@redhat.com>