vt/wrappers: add the $(EXEEXT) extension to the installed symbolic links

(cherry picked from commit open-mpi/ompi@6af465f12da4e1b223ddba9e4ab9e73e2010ace0)
Merge pull request #61 from ggouaillardet/topic/vt_wrappers_exeext

vt/wrappers: add the $(EXEEXT) extension to the installed symbolic links