Add a --with-consumerd-only configure option

This will only build the consumer daemon (and its liblttng-consumer
library), and skip the lttng and lttng-sessiond binaries.

This will be useful when a user wants to support both 32-bit and
64-bit applications on the same system. Only one sessiond is
needed, but we need a consumer for each architecture.

Signed-off-by: Alexandre Montplaisir <alexandre.montplaisir@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>