Use tools::find_gs_cmd()

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/tm@1323 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Fix example

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/tm@1324 edb9625f-4e0d-4859-8d74-9fd3b1da38cb