Headers (task_scheduler_init.h, tbb_stddef.h) removed from OneAPI TBB 2021.1

Summary:

task_scheduler_init.h is removed from OneAPI TBB 2021.1. The corresponding include should be removed from init_threadpool_tbb.hpp.

Description:

I am using a system TBB library (OneAPI TBB 2021.1). The RStan installation bug checks:

/Users/anirbanmukherjee/Library/R/4.0/library/StanHeaders/include/stan/math/prim/core/init_threadpool_tbb.hpp:12:10: fatal error: 
      'tbb/task_scheduler_init.h' file not found
#include <tbb/task_scheduler_init.h>

task_scheduler_init.h is unavailable as it has been removed from the latest TBB release. See: https://software.intel.com/content/www/us/en/develop/documentation/tbb-documentation/top/intel-threading-building-blocks-developer-reference/appendices/deprecated-features/outdated-and-redundant-api/taskschedulerinit-class.html. I have checked the folder and the header is not that there.

It should be safe to simply remove this include as the header was deprecated a while ago.

R Version:

> R.version
               _                                          
platform       x86_64-apple-darwin20.3.0                  
arch           x86_64                                     
os             darwin20.3.0                               
system         x86_64, darwin20.3.0                       
status         Patched                                    
major          4                                          
minor          0.4                                        
year           2021                                       
month          03                                         
day            18                                         
svn rev        80099                                      
language       R                                          
version.string R version 4.0.4 Patched (2021-03-18 r80099)
nickname       Lost Library Book

> sessionInfo()
R version 4.0.4 Patched (2021-03-18 r80099)
Platform: x86_64-apple-darwin20.3.0 (64-bit)
Running under: macOS Big Sur 10.16

Matrix products: default
LAPACK: /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] compiler_4.0.4 tools_4.0.4  

Open init_threadpool_tbb.hpp and inspect the include of tbb/task_scheduler_init.h. Remove the unavailable header reference, then verify that RStan installs or builds successfully with OneAPI TBB 2021.1.