Fix LdFlags for new RcppParallel

[jeroen]

Summary:

This copies a small tweak from RcppParallel to support cross compiling, see: https://github.com/RcppCore/RcppParallel/pull/208

Basically we check $TBB_LINK_LIB before checking $TBB_LIB which is needed when the runtime arch (TBB_LIB) is different from the one we are compiling for (TBB_LINK_LIB)

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Jeroen Ooms

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: GPLv3 (https://opensource.org/licenses/GPL-3.0)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[andrjohns]

Failures are unrelated to these changes, thanks again!

[andrjohns]

@bgoodri can you push your CRAN changes to the repo? That will resolve the current CI failures

[bgoodri]

Yes

[andrjohns]

@bgoodri quick reminder to please push your changes