Change quotes to single as backquotes execute automatically

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/kinfit/pkg/mkin@155 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Vignettes are not built from the makefile

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/kinfit/pkg/mkin@156 edb9625f-4e0d-4859-8d74-9fd3b1da38cb