Fix: check for dtrace and sdt.h before enabling SDT uprobe tests

Add a configure switch '--enable-sdt-uprobe / --disable-sdt-uprobe', the
default behavior of enabling the test if the requirements are found is
kept but it's now possible to explicitly disable it.

Also add the detection of the dtrace binary and its override trough the
DTRACE environment variable.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>