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

This is a backport of commit open-mpi/ompi@eef7590e585fbdaaadf90d9f890b32367e5320e0
Merge pull request #60 from ggouaillardet/topic/wrappers_symlinks_exeext

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