fix PR#15439

git-svn-id: https://svn.r-project.org/R/trunk@63802 00db46b3-68df-0310-9c12-caf00c1e9a41
Another bug fix for r63792: NMOF now builds, but vignettes that are only in inst/doc are not considered to be vignettes.  

git-svn-id: https://svn.r-project.org/R/trunk@63805 00db46b3-68df-0310-9c12-caf00c1e9a41