make: build libcommon in subdir

- DEFAULT_INCLUDES contains the current directory, which causes the
includes of common/errno.h and common/signal.h to clash with system
headers. This just remains them with a ceph_ prefix.

- Adds ceph_ver.h to BUILT_SOURCES to cause it to be generated early on
so it is available to common/Makefile.am

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
make: build librados in subdir

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
make: build libos as libtool in subdir

Builds libos as a libtool library so we can record dependencies with
automake, rather than manually managing them with the LIBOS_LDA
variable.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
make: build libosd as a libtool library in subdir

Make a libtool library so we can encode dependencies.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>