The definition is derived from pkg-config as discussed in:

  https://lore.kernel.org/lkml/20210610155915.20a252d3@oasis.local.home/

The definition is computed using expr rather than passed to be computed
in C code, this avoids complications with quote  in the variable
expansions.

For example see the target python/perf.so in Makefile.perf.
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/lkml/20210923001024.550263-2-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>