tracetool: carefully define SDT_USE_VARIADIC

tracetool: carefully define SDT_USE_VARIADIC

The dtrace backend defines SDT_USE_VARIADIC as a workaround for a
conflict with a LTTng UST header file, which requires SDT_USE_VARIADIC
to be defined.

LTTng UST <lttng/tracepoint.h> breaks if included after generated dtrace
headers because SDT_USE_VARIADIC will already be defined:

  #ifdef LTTNG_UST_HAVE_SDT_INTEGRATION
  #define SDT_USE_VARIADIC <-- error, it's already defined
  #include <sys/sdt.h>

Be more careful when defining SDT_USE_VARIADIC. This fixes the build
when both the dtrace and ust tracers are enabled at the same time.

Fixes: 27e08bab94f7c6ebe0b75938c98c394c969e3fd8 ("tracetool: work around ust <sys/sdt.h> include conflict")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20200729153926.127083-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>