optional package dependency

the Windows checks for CRAN run with that setting, i.e.

 _R_CHECK_FORCE_SUGGESTS_=false

Hence the multicore issue mentioned below actually does not exist.

I did not know that the Windows checks for CRAN used this setting.
My concern was initiated by a Bioconductor package developer wanting to
use multicore and I mistakenly thought the issue would exist for CRAN as
well.  Bioconductor currently uses the default configuration for check
on all platforms.  For the CRAN case, there is no immediate problem.

While there isn't an issue at hand, the approach still seems lacking.
What happens when there is a Windows only package that folks want to
optionally use?  Perhaps public repositories should then not force
suggests for any platforms (do they already?) -- I think that is a
reasonable and simple solution.  But in that case, perhaps the deafult
value should change.

+ seth