update makevars for CRAN

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rprotobuf@752 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
changes for CRAN win builder

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rprotobuf@753 edb9625f-4e0d-4859-8d74-9fd3b1da38cb