Add lcbcore as an 'object' library

This way we don't need to compile the object files twice for
libcouchbase.so and libcouchbaseS.a

Change-Id: Ice6856964510c24bc675aad05fa6f1284c88d120
Reviewed-on: http://review.couchbase.org/42770
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>