lttng: Don't build trace libraries when lttng is disabled

Signed-off-by: Adam Crume <adamcrume@gmail.com>
lttng: Test for sane lttng-gen-tp

Signed-off-by: Adam Crume <adamcrume@gmail.com>
lttng: Check that the main babeltrace header is present

Signed-off-by: Adam Crume <adamcrume@gmail.com>
lttng: Check for babeltrace when autoconfiguring lttng

Signed-off-by: Adam Crume <adamcrume@gmail.com>