tracepoints: print debug message when lttng-ust-tracepoint.so is not found

Rather than silently disable tracepoints when lttng-ust-tracepoint.so is
not found in the library search path, print a debug message when either
the compile unit including tracepoint.h has defined LTTNG_UST_DEBUG, or
when the LTTNG_UST_DEBUG environment variable is set.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I909f9b8dcd0cbc5faae3789931883f1941973207