Fix: make dist fails on sdt-probe test with autotools v1.16+

The 'dist' target fails when building with autotools 1.16.1 as the
behaviour of BUILT_SOURCES has changed in 1.16 [1].

BUILT_SOURCES are now built as part of the 'dist' target which causes
it to fail on machines that don't have dtrace installed.

The following error is produced: DTRACE foobar_provider.h /bin/sh: -s:
  command not found

The BUILT_SOURCES is skipped for build configurations where the build
of the test helper 'userspace-probe-sdt-binary' is disabled.

The resulting artifact, foobar_provider.h, is not part of the
distributed tarball (marked as nodist), so this doesn't have any
adverse effect.

[1] https://www.mail-archive.com/automake@gnu.org/msg20156.html

Signed-off-by: Michael Jeanson <michael.jeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ibba4689775a8b002e8a959079b1f3c8db9917c5f