Logging: define `BT_LOG*_SUPPORTED` when logging is supported

Logging: define `BT_LOG*_SUPPORTED` when logging is supported

Rely on those new `BT_LOG_SUPPORTED`, `BT_LIB_LOG_SUPPORTED`,
`BT_COMP_LOG_SUPPORTED`, definitions instead of the header include
guards to know whether or not logging is supported at a given point.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I4d076e30b528882d036f7cfc367b5dbb39401dda
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1472
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>