perf tools: Propagate CFLAGS to libperf

perf tools: Propagate CFLAGS to libperf

Andi reported that 'make DEBUG=1' does not propagate to the libbperf
code. It's true also for the other flags. Changing the code to propagate
the global build flags to libperf compilation.

Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20191011122155.15738-1-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>