be more lenient in speed test for str()


git-svn-id: https://svn.r-project.org/R/trunk@62970 00db46b3-68df-0310-9c12-caf00c1e9a41
modification of r62963: install group-writably  iff the library is g+w

git-svn-id: https://svn.r-project.org/R/trunk@62971 00db46b3-68df-0310-9c12-caf00c1e9a41