bug fix for downViewport() when descending more than one viewport

git-svn-id: https://svn.r-project.org/R/trunk@66891 00db46b3-68df-0310-9c12-caf00c1e9a41
do not warn on OS X system headers

git-svn-id: https://svn.r-project.org/R/trunk@66892 00db46b3-68df-0310-9c12-caf00c1e9a41
tweaks

git-svn-id: https://svn.r-project.org/R/trunk@66894 00db46b3-68df-0310-9c12-caf00c1e9a41
more comments about OS X Yosemite

git-svn-id: https://svn.r-project.org/R/trunk@66896 00db46b3-68df-0310-9c12-caf00c1e9a41