clarify

git-svn-id: https://svn.r-project.org/R/trunk@67243 00db46b3-68df-0310-9c12-caf00c1e9a41
add check for GNUisms in Makevars

git-svn-id: https://svn.r-project.org/R/trunk@67245 00db46b3-68df-0310-9c12-caf00c1e9a41
add note

git-svn-id: https://svn.r-project.org/R/trunk@67246 00db46b3-68df-0310-9c12-caf00c1e9a41