tweak header check

Author: kevinushey

src/options.cpp

@@ -10,7 +10,7 @@
 
 extern "C" SEXP defaultNumThreads() {
    SEXP threadsSEXP = Rf_allocVector(INTSXP, 1);
-#ifndef __TBB_tbb_stddef_H
+#ifdef __TBB_task_arena_H
    INTEGER(threadsSEXP)[0] = tbb::this_task_arena::max_concurrency();
 #else
    INTEGER(threadsSEXP)[0] = tbb::task_scheduler_init::default_num_threads();