The previous entry set env RCPP_PARALLEL_USE_TBB=0, but RcppParallel reads that
only as a compile -D flag, not from the environment — so it was a no-op and the
bundled Intel TBB build still ran (and hung on musl / newer toolchains).
Replace it with a source patch on src/Makevars.in that leaves USE_TBB unset
(skipping the bundled TBB build) and forces -DRCPP_PARALLEL_USE_TBB=0 so the
sources use the TinyThread backend. Verified in a Linux container: RcppParallel
installs with no bundled-TBB build (* DONE (RcppParallel)).
