configure: fix VIM_DIR value when PREFIX is set
configure: fix VIM_DIR value when PREFIX is set

fxfactorial/merlin,neojski/merlin,chambart/merlin,ocaml/merlin,dra27/merlin,the-lambda-church/merlin,chriskonstad/merlin,haselwarter/merlin,neojski/merlin,the-lambda-church/merlin,chambart/merlin,chriskonstad/merlin,rgrinberg/merlin,ocaml/merlin,chambart/merlin,dra27/merlin,haselwarter/merlin,fxfactorial/merlin,dra27/merlin,haselwarter/merlin,fxfactorial/merlin,chambart/merlin,dra27/merlin,rgrinberg/merlin,neojski/merlin,ocaml/merlin,haselwarter/merlin,neojski/merlin,fxfactorial/merlin,rgrinberg/merlin,the-lambda-church/merlin,rgrinberg/merlin,ocaml/merlin