tbb: Backward compatibility based on __TBB_tbb_stddef_H

`__TBB_tbb_stddef_H` is defined in `tbb/tbb_stddef.h`, which has been removed in the new TBB interface.

Author: hsbadr

src/options.cpp

@@ -10,7 +10,11 @@
 
 extern "C" SEXP defaultNumThreads() {
    SEXP threadsSEXP = Rf_allocVector(INTSXP, 1);
+#ifndef __TBB_tbb_stddef_H
    INTEGER(threadsSEXP)[0] = tbb::global_control::default_num_threads();
+#else
+   INTEGER(threadsSEXP)[0] = tbb::task_scheduler_init::default_num_threads();
+#endif
    return threadsSEXP;
 }