[Rd] Why not pthreads on Windows in 'parallel' package?

[Henrik Bengtsson]

On Windows there are a few 'pthreads' implementation, e.g.
pthreads-w32 and winpthreads
[https://cran.r-project.org/doc/manuals/r-devel/R-exts.html#Using-pthreads].
We're thinking of giving them a try for the matrixStats package, and
basic tests indicates it works, but since Windows pthreads are not
used by core R (or?) I've got a little bit worried that we will face
overwhelming problems.

So, why are the above Windows implementations not used in the
'parallel' package in order to add multicore support for mclapply() on
Windows?  Was it tried but found to be unreliable?  Was it that no one
had the time to do it?  License issues?  Are there any pointers to old
R-devel threads discussing this?

Thanks

Henrik