configure: allow one to override the version string returned by merlin
configure: allow one to override the version string returned by merlin

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