message: enable disabling Linux perf usage through PCM_NO_PERF env variable
  export PCM_NO_PERF=1
  no recompilation needed to disable Linux perf usage
diff: @@ -1852,6 +1852,12 @@ PCM::ErrorCode PCM::program(const PCM::ProgramMode mode_, const void * parameter
  #ifdef PCM_USE_PERF
  std::cerr << "Trying to use Linux perf events..." << std::endl;
  + const char * no_perf_env = std::getenv("PCM_NO_PERF");
  + if (no_perf_env != NULL && std::string(no_perf_env) == std::string("1"))
  + {
  + canUsePerf = false;
  + std::cout << "Usage of Linux perf events is disabled through PCM_NO_PERF environment variable. Using direct PMU programming..." << std::endl;
  + }
  if(num_online_cores < num_cores)
  {
  canUsePerf = false;
  